Have you got any tips on four-part writing (preferably on a grade 5 music theory level)?

Yes but what sort of tips would you like?

Let's assume you are writing a string quartet for example. Again assume you have a basic melody and harmonic (chord) progression.

From here there are any number of things you could do. The simplest is to have the 'cello playing your bass line which should primarily be the root note of the harmonic progression but you can add some harmonic flavourings as well. Keep it reasonably simple.

Use your viola and 2nd violin to flesh out the harmony again this is up t you what sounds best. A simple starting point would be to have the viola playing fifth intervals with the 2nd violin playing non-perfect elements of the harmony (thirds, sixths etc).

Carry your melody line in the 1st violin by and large.

TO make the piece more interesting introduce some antiphony (question and answer) passages between the instruments. You can use the different tonal ranges of the instruments to create different textures in this regard. For example you could have a short meoldic theme starting in the 'cello which is echoed by the viola and finally by the violins so you create an ascending pattern which could add some nice tension to your piece.

Try incorporating some pedal elements. Either the 'cello playing pedal notes or perhaps the 1st violin playing an inverted pedal whilst maybe the viola takes the melody for a time.

Regarding harmony decide whether you are going to go for modal harmonies or not. Modal can add some nice flavouring but can also get out of hand and become confusing. Best to stick to using notes within your key signature unless you are performing temporary modulations and so on.

And so on. There are so many things you could try.

Please feel free to ask more specific questions if needed as this is a bit of a vast topic.

There are a lot of tips. You should look at some books. Here are a few of the most important. (1)Write within the correct ranges. For vocal music, make sure all four parts can be sung comfortably by a SATB singer. For instrumental music, keep the parts within the practical ranges for each instrument. This would require an knowledge of each instrument you are writing for. (2)Try to have the "moving parts" vary between each of the four parts. Don't put all the movement in part one and then have the other 3 parts just playing chords. (3)Make sure each phrase resolves nicely to the beginning of the next phrase in all parts. The effect is lost if one part has a clever resolution and the other parts don't do anything interesting or important. (4)I like to use contrary motion as opposed to parallel motion. Have the contray motion move from part to part in each new measure. Likewise for parallel motion - don't keep it all in the same parts, move it around. (5)Learn to look at the music "vertically," more so than horizontally. Use interesting harmonic structures (chords) and proper resolutions from one chord to another. (6)Listen to other examples of the type of music you are writing. Analyze what you hear and learn from it.

1) This could be interesting:
"GARRITAN INTERACTIVE
PRINCIPLES OF ORCHESTRATION
by Nikolay Rimsky-Korsakov"
http://www.northernsounds.com/forum/showthread.php?t=48158


2) "Four-part writing
Four part chorale writing is used to teach and analyze the basic conventions of Common-Practice Period music. In the German musicology tradition referred to as functional harmony. Johann Sebastian Bach's four voice chorales written for liturgial purposes serve as a model for students. These chorales exhibit a fusion of linear and vertical thinking. In analysis, the harmonic function and rhythm are analyzed as well as the shape and implications of each of the four lines. Students are then instructed to compose chorales, often using given melodies (as Bach would have done), over a given bass line, or to compose within a chord progression, following rules of voice leading. Though traditionally conceived as a vocal exercise for Soprano, Alto, Tenor, and Bass, other common four-part writings could consist of a brass quartet (two Trumpets, French Horn, and Trombone) or a string quartet (including violin I, violin II, viola and cello).
There are seven chords used in four-part writing that are based upon each note of the scale. The chords are usually given Roman Numerals I, II, III, IV, V, VI and VII to refer to triadic (three-note) chords which are based upon each successive note of the major or minor scale which the piece is in. Chords may be analyzed in two ways. Case-sensitive harmonic analysis would state that major-mode chords (I, IV, V7, etc.), including augmented (for example, VII+), would be notated with upper-case Roman numerals, and minor-mode chords, including diminished (ii, iii, vi, and the diminished vii chord, viio), would be notated with lower-case Roman numerals. Schenkerian harmonic analysis, patterned after the theories of Heinrich Schenker, would state that the mode does not matter in the final analysis, and thus all harmonies are notated in upper-case.
The skill in harmonising a Bach chorale lies in being able to begin a phrase in one key and to modulate to another key either at the end of the first phrase, the beginning of the next one, or perhaps by the end of the second phrase. Each chorale often has the ability to modulate to various tonally related areas: the relative major (III) or minor (vi), the Dominant (V) or its relative minor (iii), the Sub-Dominant (IV) or its relative minor (ii). Other chromatic chords may be used, like the diminished seventh (made up of minor thirds piled on top of each other) or the Secondary dominant (the Dominant's Dominant — a kind of major version of chord II). Certain standard cadences are observed, most notably IIb7 – V7 – I. The standard collection of J. S. Bach's chorales were edited by Albert Riemenschneider and this collection is readily available, e.g. here; the student is greatly rewarded by playing them at the piano, singing the lines by themselves, singing them in groups, analyzing them by writing the Key and the Chords employed and by taking the melody and bass line from any chorale and trying to fill in the inner alto and tenor parts. Once this has been accomplished the student can then begin to complete their own bass lines —whilst carefully watching for modulations— and then they can fill in the inner alto and tenor parts. Parallel octave and fifth motion is forbidden, and this often proves to be the pons asinorum of the average music student.
Source and further information:
http://en.wikipedia.org/wiki/Music_theory#Four-part_writing


3) "USE PROPER NOTATION.
Notate the soprano on the treble clef staff with stems up, the alto on the treble clef
staff with stems down, the tenor on the bass clef staff with stems up, and the
bass on the bass clef staff with stems down."

"Chords may be in open or close spacing. Chords are in open spacing when you
can put another chord tone in between the soprano (S) and alto(A) and/or
between the A and tenor (T) voices. Chords are in close spacing when no
chord tone can be placed between the SA or AT voices."

"• MAINTAIN 4 DISTINCT VOICES; MAINTAIN THE INDIVIDUALITY OF THE VOICES.
Keep the voices in their proper ranges.
Soprano Alto Tenor Bass
The maximum distance between SA and AT is a P8; between TB, the maximum
distance is 2 P8s.

Do not cross voices (in the same chord). Do not overlap voices (between two
adjacent chords).

Use contrary or oblique motion to the bass more frequently than similar motion.
Use parallel motion only when necessary.

* Contrary motion -- voices move in opposite directions.
* Oblique motion -- one voice remains on the same note while the other
voice moves.
* Similar motion -- voices move in the same direction by different
intervals.
* Parallel motion -- voices move in the same direction by the same
interval.

NEVER write parallel P1, P5 or P8."
Source and further information:
http://web.utk.edu/~mtheory/documents/Murphy_110_PartWritingRules.pdf