I really enjoy turning 4140 on my lathe, it gives an amazing surface finish and seems to machine pretty easily. As for welding, IIRC 4140 requires some specific welding techniques to prevent it from cracking during the cool down period, totally do able, but you might want to do a bit of research if you haven't already. In a perfect world, with TIG welding, the metal will expand during welding but then contract back to original size (I think) after cool down. Might want to do a few test pieces first to get the hang of it and learn your mistakes, that's always a good idea. Best of luck!