My Poulan 2775 is basically the same saw as the 4620. It is presently muffler modded, ported and has the timing advanced and runs like crazy. Even when it was just muffler modded it had plenty of power. They have a couple of weak points - the engine mount bolts can back out and the rear A/V spring loosens, especially if it gets pinched and you yank on it. That said, it is a light, narrow and well balanced saw and I really like mine. A loop of 20BPX turns it into a totally different saw than with the 33SL it came with.
I don't have the 445, but I have run my Dad's 455. You'd get a newer design with a strato engine, but it's still a plastic chassis saw and unlikely to add much to your capability. Why not look at a saw of different size and keep the 4620?