Just slow down any income by 0.3 or 0.4. Any, MP, Ammo, Fuel and caches.
So instead of KT 15 min mark, you have a KT at 25/30 min mark if you can survive that long without investing in midgame units.
3vs3 and 4vs4 are actually too fast, faster than 1vs1 and 2vs2. We all know that.
With the current meta, unit path + moment of fight + teching are simply not fast enough to drain any MP/Ammo/Fuel income. Not like in 1vs1 or 2vs2.
Many time, you build your force, you move to the choke point, you fight and the moment your retreat to reinforce, you're already full of resources to reinforce all of your squads, full of ammo to use your special abilities as you want for the next fight and fuel to build your next tank.
So let's use some wording.
You have:
1- Time To Fight (comprise between the moment you leave your base to the moment to finish your fight)
2- Time To Reinforce (comprise between the moment you finish your fight and the moment your finish to reinforce - at your base or on the field)
3- Time To Get Resources to reinforce. (time for your income to naturally reach the amount to fully reinforce your available squads)
Bigger the map is, bigger the TTF and TTR are, and in its way also TTGR since there is more resource points to cap in 3vs3 and 4vs4 maps.
If your TTF + TTR = TTGR, every time you fight and retreat to reinforce, you will find enough resource to reinforce all your army and come back to fight with the only TTR delay. This is pretty much the situation we are facing today in 3vs3 and 4vs4.
If now you reduce the TTGR by 0.3 or 0.4, TTF + TTR will be higher than TTGR.
Consequences:
1- You will immobilize your force at your base more time between fights. Which is not good in term of strategy. This may force people to blob less, or to build less units to reduce your upkeep consumption. Or to play more conservative, here again less blobbing and using more covers.
2- You will have to make more decisions between reinforcing and leveling. If you have less income, you'll need to decide when you want to reinforce and when you need to tier up your army. So you may decide to keep going a bit more at a lower tier or to tier up. But if your opponent successfully tier up, you'll have to counter him. If you don't you'll lose more resource by reinforcing more your losses delaying more your teching. If you tier up too fast, you may be overwhelmed by your opponent by lack of units.
So there it is, in conclusion, lowering any income (and not only caches) will simply force people to make strong decisions and assume them since any loss will have a biggest impact on his army.
In my opinion, it would simply put it at 1vs1 and 2vs2 speed. So nothing extremely wrong