Well maybe so but there are no such thing as an obsolete weapon, you just have to change the tactics. You might not be able to go head to head (unless you “Zerg Rush” them perhaps) but you can attach from the sides and rear. A side note, I’ve seen many debates about the King Tiger vs. M1 tank.
attach=attack
Tactics for using M4s with 75mm main guns against T-72BMs or T-90s with 125mm main gun, both with associated infantry: bail and run. Out gunned and out ranged. Not to mention those pesky machine gunners with armor piercing depleted uranium rounds ...
Side/rear/swarm tactics worked with M4 vs Tigers assumes that Tiger is isolated/undefended and either stationary or broken down, and has slow rate of fire.
T-90s (and I think the T-72BM variant both have auto-loaders), with the T-90 boasting 3 rounds in 13 seconds. Thus, every minute one T-90 would take out 7 or so M4s. Unless you massed all 5000 in one battle, the battle would be over very quickly and likely all Russian tanks would still be standing and mobile.
Just barely mentionable is manual M4 targeting verses T-90 computerized targeting ...
King Tiger = maintenance beast/garage queen. Too few ever made. 88mm round probably would ‘bounce’ off M1A2’s or T-90s reactive armor.
But then, you didn’t offer the Ukrainians any King Tigers either ... Panthers would be a better offer, as there are still some in use or were until recently. Ask around in Poland.
5000 M4 = target practice for T-72BM/T-90. Given 300 more MBTs just crossed that would be 16 and change apiece for the Russians and zip for the M4s.
You need to break the piggy bank/start a fund raiser and dig deep to spring for some German Leopard IIs or at least some Israeli Merkava 4s.
Be sure to make some combat videos of the battles - inquiring minds and all.