If i join 4 para and decide i want to be regular and do the job fulltime ie. 2&3 para what would the process be?

There are many people who look at doing either a Regular to Reserve transfer or from Reserves to Regular transfer. For the latter it's a bit more complicated and would depend on a huge number of factors including time spent in the Army Reserves, courses completed (ie Phase 1 and 2 training), whether there is space (number of personnel etc) etc etc. It is a faster process than starting from scratch as you are serving in the Army albeit as a Reservist but all the medical etc is already completed. If you join 4 para and later decide to become a Regular there will be plenty of directing staff in 4 Para who will guide and help you through the process. Good luck with whatever you decide to do.