See how much effort it takes to destroy this building? Not to mention, the map has a huge influence on how Allied players can handle this situation. Add this to compounding problem of committing 25% of your troops for this task while Axis player is free to roam the map and do all kinds of damage (map control, ambulance hunt & flanks). Lastly, the areal denial on this truck makes it difficult to pull off mid game (he/she can sit on a fuel point until then). The whole point of this truck is to keep opponents from cutting off OKW (slap in the face for COH2 tactical play).
Then do what you should and kill it's T2 (meaning jagdpz building) before he even tries to establish his T4. Coordination it's a sweet thing in team games and it's not so hard to catch a T2 deployment or a T2 advanced building with a united push. While being OKW, your other mate won't stay there for ever to protect you untill you get stronger. I lost my T2 while playing OKW several times due to good coordination and tactics of allied players. Then things became worst for me of course. That's all about allied game in early stage: harass and push, especially when there is an OKW player on map.
Again: it's not hard to destroy OKW t4 for reasons i explained in my previous posts. In fact, it's quite hard to efficiently place the t4 because you should cover 2 things:
1. put it somewhere where it will mean something (where it can be usefull);
2. deployed it in a place where is not to exposed in order not to lose it or lose it to quiclky while it can STILL provide support.
I studied these problems from USF, OKW and Soviet perspective and it's not easier in anybody's shoes.