Mods to 4AGE depend on how much money you want to spend and what type of 4age it is.
Don't know what kind of car its in but I'll assume its standard.
Start with suspension and ensure everything is right for you such as height, stiffness and damping rates etc etc.
Engine wise start with intake and exhaust. If you have a pod filter make sure you have a decent cold air intake. Hot air from engine bays suck power.
Exhaust-wise, you probably won't need extractors as the general consensus is that the factory headers are quite good. For a 1.6 4 cyl you won't need anything bigger than 2 to 2.25 inches.
As for internal mods like cams you can't modify too much with the factory computer. Go to Bill Sherwoods site and check it out. He knows pretty much everything there is to know on modifying these engines.