"Do you think the single shop light with 2 tubes and give enough light for all 4 flats of seedling?"

Oh, no. No it won't.

You'll get weedy seedlings, unless you happen to be growing in front of a southern window.

If you have a Home Depot nearby, there's supposed to be a sale going on for their Commerical brand of shoplight, normally $7.25 now $6.35

How big are your flats? If they are narrower than the double shop light and shorter than 4' when laid end to end, then you'll gain very little with more tubes because you won't be able to fit them in.

Also, what seeds? Most annuals and veggies will take as much light as you can give them from a fluorescent, which means lamps across the full length and width of the seed trays, as close to the seeds as is practical, plus at least a simple baffle or reflector above the tubes.

However, many types of seeds don't need so much light, including most seeds for indoor type plants, some orchids, cacti, and many succulents.