4 star units aren't all bad. There are four star tanks that don't have a five star role equivalent, like the M8 Scott and the Crocodiles.
Try to capture cities as soon as you can. It's the only way to get currency that's not phenomenally tedious. Currency is one of those things that's debilitatingly scarce early on and then (presumably) becomes over-abundant once you've maxed out all your emblems and bought all your scouting slots and so on.