I've had a mess with the WRC4 demo and have to say, the presentation is a massive leap over WRC3. It's definitely looking like it's now the full console version on Vita unlike last year's offering. There are definitely more courses.than WRC3 has, but given last years Vita port had some missing that's to be expected.
The music is also a lot better than what was on offer last year.
Graphically it doesn't look too different. I don't think the demo stage really shows the game off too well as it's set at dawn and overcast so it just makes everything look dull. Car models however look much much nicer. Crisper textures and improved reflections.
Definitely looks like WRC4 is going to be the one to go for if you were considering WRC3. Unless you're a massive WRC nut though I'd probably hold off until it's a bit cheaper.