The only aircraft that I could see lifting four Abrams would either be an airship or a ground-effect aircraft like the Russian Ekranoplans or the Boeing Pelican. http://en.wikipedia.org/wiki/Boeing_Pelican Such an airship would have to be really, really huge. It takes a cubic meter of hydrogen to lift 1.2 kg of weight, so just lifting four abrams would take a hydrogen-filled airship with the same volume as the 800-foot-long Hindenburg. Then you'd need even more hydrogen to lift the structure, engines, etc. There are two major problems with a jet-powered VTOL of that size. The first is that it will require an absurd amount of fuel for takeoff and landing. The second is what you will take off and land on - the much smaller Harrier can't take off and land immediately from the same patch of runway due to heat, and the Navy has had to reinforce their ship's decks to deal with the heat of the Osprey's engines. For VTOLs I think one IFV or light tank is the limit. Even the C-130 size Mil Mi-26 can only carry 20 tons, and nobody else has ever bothered to make a real operational helicopter (ie not the V-12) anywhere near that size.