We have a 72" vanity with two lights over each sink and they are fine. I think you could do 3 lights over each without it looking crowded, but I don't know about 4 lights. Does each light have a shade or are all the lights behind a single shade?

We have that also...30" cabs on each end. I don't know what kind of mirrors you're doing over each sink. But if they're coming in from the wall a little...say two inches on each side, that's bringing your mirror width to 26". You may also want the lights to come in a little also.

Personally, I'm doing double sconces over our sinks. I like the look this gives better and there's plenty of light with the other lighting in our bathroom.

Some images for you to peruse: