Can you extend your service contract before your time is up?

[Jordan]

When I graduate from AROTC I will owe 4yrs active, 4yrs reserve (I have the 4yr scholarship). If I decide that I like the army alot after 3 years or so, can i choose to sign a new contract for a couple years with the army or do I have to wait 4 or even 8 years? Also, when I get my branch coming out of ROTC, do I have to be that branch for my whole service obligation, or can I try out for special forces, or switch to another branch?