One thing you should be aware of, which is really quite difficult playing-wise: the string quartet is, of all western ensembles, perhaps the one where there's most non-trivial work done in intonation. There is no way you can achieve a good, "proper quartet" result when just playing all the notes in equal temperament as you would on piano (and might get away with for larger orchestral sounds), rather you need to tune every single note to either some just interval so it matches up with the other instruments, or some melodically expressive scale akin to a Pythagorean one.
There is no fixed reference to tell you which fine-tuning to apply to each note a priori – actual quartets establish this during years of practise together. I suppose if you've never done this before the best you can do is first record everything in MIDI and then add pitch-bend information to it later. It's going to be a heck lot of trial-and-error work, though.
Also, string instrument players add of course all kinds of other expressive playing details. Vibrato may be the most obvious; that's in principle easily introduced with something LFO-like, but especially for string quartet this is problematic because real quartets again sync their vibrato in most non-obvious ways. Similar applies to dynamics. Articulation like spiccato is also pretty hard to replicate on a keyboard instrument.
Finally there's sound details that aren't necessarily done on purpose, but aren't generally surpressed either by strin players, and may be considered necessary for a real authentic replication of quartet sound. For instance, cello position shifts retain always a very slight slur quality to them.
In summary: if you want it to sound like a real string quartet, employ a real string quartet! It may nowadays be possible to replicate the results perfectly with digital means, but in particular for a small setting like quartet it's not likely to be worth the effort, because effectively you're going to replicate much of the work that each string player has already gone through.
If you're nevertheless determined to give it a try, I'd recommend starting with a good expressive first violin, 8dio solo studio violin is the best I've heard so far (but I haven't tried it myself). Take your time to learn the expressive features, then record the 1stVln track (perhaps over a piano rendition of the full quartet). After that, add the other instruments.