Need to change ballast in fluorescent fixture. Old ballast was 4 wire, red, blue black and white - New ballast has 2 reds, 2 blues, 2 yellows, a black and a white. Now what?

Sounds like u got the wrong ballast.

Chuck

Sounds like you have a 8 foot 2 lamp fixture using Slimline lamps that have a single pin at each end, and a ballast for a 2 lamp 4 foot fixture that uses a two pin lamp.

Some detail of what you have for lamps and old and new ballasts, such as make and model numbers would be helpful. Otherwise we are just guessing.

Could be a high output 8' ballast. What does the ballast say on it, wattage and bulb type?

Many ballast have a small diagram on the label.