Hi Bonnie
The most important thing is what city do you live in?
Each market is different, and the time to buy changes with the market. Also each market has colors that bring more money then others.
Yes the new C6 is coming out and it will hurt the c5's more then the c4's. The c4's are cheap to start with, but this time of the year they are peaking now.
You will also notice that vette's will vanish in Nov and resurface in April. Any decent vette that is for sale privately between those month's is a party that has to get rid of it for some reason.
How much money money do you want to spend and how much money are you willing to put into it within 60 days after buying one.
This will determine what year you can start looking for. The 96 is the best year of the c4's, but a low mileage, mint 94 would be the better choice.
Do not wait for a specific time, first find a vette that fits your budget and condition and then deal with it.
Do not settle on a color.
Don't forget that age old saying " sometimes saving a dime can cost you a quarter".
Replacement parts are very expensive, so watch the tires, brakes, interior (seats, carpeting) and so on.
You cannot buy a vette without a qualified vette mechanic, not just a regular mechanic (not knocking regular mechanic's), even if you buy it from a dealer.
Alan
p.s. Good Luck