If wishes were horses then paupers would ride. My wish is that the 400 were the size and weight of the 300. In that case I'd have gone 400 in spite of cost. Having now carried the 300 plus TC around quite a lot I can't say I'd wish to carry the 400, especially while jumping over little streams and ducking bushes etc.
However, when I'm hiding with aim on that distant bird with 300 X2 I'm always wishing for more (not to mention when comparing to those unbelievable shots with the 600 prime we see on the forum, compared to 300 X2). Untill my arm starts to ache that is. Obviously then use a tripod. But that's more to pack and hook on branches along with the 3 legged stool and so it goes.
Ultimately you make the best of what you've got.
In spite of the above, the 300 has provided me with the thrill of a lifetime already and I don't regret having made the purchase and neither will you regret your 400 or other big white.