I would highly recommend getting a sub, even if its a 10", they dont take up much space and some of the sub + amp packages about are really good.
SPL-Dynamics Comps £78
Sub + Amp £117 - cheap and in a compact enclosure
Amp for components £63
Thats £258 and i highly doubt you'll get much better for that.
if your new to the audio game, start off with a sub and you will be amazed the difference it makes, it doesnt have to be like a typical BOOMBOY sub where your car is rattling apart from the bass. set it as you like by adjusting the gain on the amp. it should be so that it complements the music and it WILL add a nice depth to it.
So a sub would be the 1st thing to get.
Edit: Just to add to that i wouldnt bother with replacing the rear speakers, when at a gig you dont stand with your back to the stage. leave the standard ones in for rear fill, good front will be more than enough
Once the sub is sorted i'd go onto a set of components up front (6.5" is the correct size in a mk4) for the first few months you could run them without an amp (just off the headunit) and you will notice a huge difference in clarity.
But i would recommend amplifying them to unleash their potential, and by doing so you will get much more midbass and a crystal clear, undistorted sound at pretty much any volume.
and i honestly would recommend the sub to start. really does make such a difference.
Most of my mates just have pure bass and it makes the look like t*ats IMO hearing them come from miles off. have it subtle and it will sound gooood.
Hope that is of some
help