If you've been through a couple different skis, probably the biggest thing that will help you is coaching from a pro. They will be able to dial you in with what you personally need to do to get to the next level. If you have been working at the same pass for a few seasons then the investment in coaching will probably do more for you than the investment in new equipment.

If you've been skiing on your ski hard for 4 seasons now, it's probably getting close to the end of its life. Skis do wear out over time. I would say dependent on materials and construction the useful life of a ski would be from 100-500 sets. However, most pros can tell the difference between a ski with 50+ sets on it versus a brand new one. The brand new ski is just more lively--however, this is not to say that the 50+ ski is "shot"-- and this probably won't effect you unless you're skiing shorter lines.

If your ski is a lot closer to the 500 set mark, it might be time to think about replacing it.

I agree with Wade and both items.  If you are looking for a new stick, consider the 69 Radar Senate C.