Just booked my indy in for a full exhaust system made by tube engineering.

Firstly i don't have a clue about exhausts, and which type is best.

So which type is best, 4-2-1, or 4-1?

The engine is a 2008 R1, and will be used for road/trackdays.

And also he mentioned a 5" cat that he can install into the silencer, will this be enough to pass IVA?

Cheers

Depends where and how you want the power delivery. 4-1s tend to produce peaky power at high rpm. 4-2-1 tend to be low-mid range spread. But many factors influence both so it's not like a one-answer fits all situation.