Your shop sounds similar in size to mine, perhaps a bit larger. I had six, four foot, two tube fluorescent fixtures and real poor light plus constant problems with the tubes and ballasts. The fixtures are the $19.99, CHEAP shop lights found in every home supply place.
I purchased some Sunco brand LED bulbs on the internet. They were not as cheap as what was available locally but I do not regret the cost. I paid around $9 per tube. I REMOVED all the ballasts and rewired the cheap fixtures for the LED bulbs and the results were spectacular.
The light is excellent. There are no dark corners: everything is well lit.
These bulbs have been installed for around two years now and I have had zero problems. Even those cheap, shop style fixtures are working perfectly. No dim bulbs. No intermittent lights. NO PROBLEMS WHAT-SO-EVER. I am delighted.
PS: I also have a lot of four foot fluorescent fixtures in my house. I replaced around 50+ tubes with 39 LEDs and the light is BETTER everywhere that I did that. All of those fixtures were around 30 to 40 years old. Absolutely zero problems in the house also. I highly recommend these bulbs - Sunco brand.
My point is, it is not the fixtures, it is the LED BULBS that count. And if you use older fixtures with ballasts, I very strongly recommend REMOVING those ballasts. You get more light (higher efficiency) and remove a potential source of problems.
You say you have four, eight foot, two bulb fixtures. You can probably just get LED bulbs for them. Remove the ballasts, rewire the fixtures for the new bulbs and put then in. Enjoy. And get some sun glasses for use in your newly lit shop.
If you want to use the four foot (48") bulbs, I would suggest replacing each of your four eight foot fixtures with two four foot ones, perhaps separated by a few feet to spread the light around better. And do not worry about the fixtures. It is the bulbs all the way. My house and shop is proof that even the cheapest and/or oldest fixtures are 100% OK.