This might of been mentioned before, but I have standard brakes on my octy 4x4 1.8t at stage 1 and they are!!!! Scary!!

What's the cheapest brake upgrade I can do?

I heard I might only need brake carriers to go to 312mm and vented on the back? Will I need quattro brakes?

May as well use the VRS setup as mentioned.  Should be decent enough

Would the vrs set up fit the 4x4?

4x4 front standard are 288 so only carriers needed to go 312mm (and discs obviously). Rear 256 discs will fit but you need quattro calipers and/or carriers as front wheel drive carriers are wrong offset for the discs. If buget allows then leon cupra r brembo setup is awesome but may need wheel upgrade and/or spacers as their 323mm.