Hi,
The TR4A gas tank is pretty easy to remove. Just a few bolts, the filler hose on the top and one hose on the bottom. There is no vent line. The vent is integral with the filler cap.
There is probably a drain, opposite side from the outlet hose, but I seriously doubt you'll get the plug out of it. Most are soldily rusted in place and I wouldn't try to hard to remove it. Note: All the fittings on TR fuel tanks are soldered in place. Don't torque heavily on them trying to loosen something or you'll end up with a much bigger repair job. (Don't ask me how I know that!)
I'd suggest pulling the rubber hose from under the tank and draining all that old fuel out of the entire system. Also look at the condition of the rubber hoses themselves. Modern fuels can wreak havoc inside older rubber hoses, causing all sorts of "mystery" problems with the carbs. Bits of rubber hose can end up working through the system. It's possible the diaphragm in the fuel pump, if an original style one is fitted, is due for replacement too.
Gas tanks can be given a new lease on life by using one of the repair kits commonly available (
www.eastwoodco.com is one source). These involve three parts: a cleaner/rust remover used first, a metal prep that's sloshed around inside the tank after that, then a sealer coating which can actually solve pinhole leak problems and helps prevent new rust from forming.
I've heard from various sources the cleaning and prep are very important, so that the sealer coating sticks well to the inside of the tank well. Otherwise, bits of it can get into the fuel lines and eventually the carbs, causing problems. Doing a tank restoration like this allows putting off buying a new tank for 5 or 10 years.
Another "trick" I've heard is to drop a length of chain into the tank and rattle it around until your arms are tired. It will knock loose surface rust. But, a good cleaning is needed afterward, and depending upon how rusty the tank is, it might then need the cleaning/prep/sealing done.
As to different brands of gas, Chevron does have a high detergent formulation that helps keep carbs and fuel injectors clean. I try to use a tank of it every so often, rather than buy a separate fuel system cleaner additive. At least here in Calif., I see Shell offering something similar.
Here Chevron tacks on about 20-25 cents a gallon for the "good stuff". The 24 gal. tank in the Land Rover only sees it every once in a while!
Have fun tinkering!
Cheers!