He has no problem holding his head up and looking around and seems very happy as soon as he's over. Lately, however, he flips and then seems to want to crawl, desperately. He lifts his butt in the air and flails his arms out to the side and starts growling and gets himself so worked up he starts crying. I should mention he can flip over to his back sometimes, and I try to flip him as soon as it starts, but he goes right back to his stomach which tells me that's where he wants to be. He really seems to be trying too hard.
Frustrations are inevitable as babies strive to learn new skills.
Your son's frustration shows that he is very motivated to attain new motor skills. Children need to be allowed to develop at their own pace and in their own way. Four months of age is not too early for infants to show the first signs of attempting to creep or crawl, even though they might not be able to master this for several more months. Some show no interest in this until later, and that is normal too. Often we adults try to spare them the frustrations that so often occur, or we hurry them along.