Good info from Oldwrench on replacing just the tubes on existing fixtures.
The tubes that work without a ballast are called bypass tubes. IMO, that's the way to go. First, if the goal is to save energy, getting rid of the ballast does that. Also, if you buy tubes made for use with a ballast, you'll find that they will generally only work with specific ballasts. The general knowledge says you need electronic ballasts, which is true, but the reality is that they won't work with all electronic ballasts.
Another point is to buy frosted tubes rather than clear ones. The clear tubes generate shadow patterns that will drive you nuts.
There's an option to convert an 8ft 2 tube fixture to use four 4ft tubes. Comes as a kit with sheet metal parts and tombstones for about 12 bucks. This was a worthwhile option when 8ft LED tubes were $40 apiece or more, but now that 8ft tubes are more like $25 each, the savings of using 4ft tubes isn't there if you consider the extra labor in adding the kit to the fixture.
Re the lumen output of fluorescents vs LEDs, you don't need LEDs with higher lumen output to have more useful light than you were getting from your fluorescents. This was explained by a lighting engineer on another forum. FL lumen output is measured under lab conditions with a perfectly matched ballast. Real world output will be in the 90%+ range with average ballasts.
A FL tube emits light 360*. With a clean white reflector, about 30% of light is lost due to less than perfect reflectivity and the fact that the tube itself blocks part of the reflected light. LEDs don't rely on reflectors and are available in different beam angle spreads.
Also, within a few months to a year, the output of a FL tube will level out at around 70% of rated lumens. Combining the 30% reflectivity and blocking loss with the 30% loss of tube output over time, and you have approximately 50 to 55 percent of the rated FL tube output available as useful light. The better LED tubes such as the Hyperikon Oldwrench mentioned are rated to put out 90% of rated lumens across their life span.
In my own home shop I've seen firsthand that an 18W 2200 lumen single LED will put out as much useful light as a 2 tube F40t12 fixture with tubes a couple years old.