Four is certainly not too old. Personally I would wait until I had moved to obtain the larger, heavier agility equipment, it really is a pain in the butt to transport. At least the regulation equipment is lol. Before you get the equipment, I would recommend purchasing some books and dvd's on training agility. You can get all you need at
www.cleanrun.com
Finding a good instructor is also very important, one that is competing now, has titled dogs, and continues to train with other top instructors etc. btw, agility is a blast.
Good luck
Lynn