Since jacksons hardcounters every vehicle, and jackson is build nearly every game..you should not build any tanks anymore?
I have problems winning the infantry game against vetted, double equipped rifles, but could be my lack of skill
Could be, as your opponent invested X amount of resources in pure AT unit and you still can't beat him in AI.
Not sure about 1v1 as I don't play it, but according to casts that I watched Pz.IV is perfectly viable in matchup vs USF. The idea of sitting on fuel just to build slow fat target and be outranged looks indeed bad.
4v4 is completely another beast. You can pretty much spam rocket artillery instead of tanks(if you see enemy is investing heavily in AT units) and be totally fine. You just need to mine flanks in order to avoid throw-away medium tank rush for your rocket artillery.