Hi! So first off, I think your skeletons are looking good! You got the poses down for the most part :] I feel like the line of action for #4 is a little off though? Like, you drew his "spine"/LOA bending outwards (towards the viewer), though in the reference picture, he's more leaning away from the viewer (just a little slouched). So I'd just be a little more careful about stuff like that O: But again, I think you got the idea of the poses down, and that's what's important!
Also, if you want to practice stuff like basic shapes and lines more, I recommend looking through the first few lessons on DrawABox.com ! The artist there gives a lot of good instruction, and it's based on Peter Han's lessons on dynamic sketching (and it's all free haha) \o/
In any case, keep it up! Doin' great!