/cracks knuckles. Let's blow this popsicle stand.
Picture One: You cleaned up the lines quite a bit and yes all in all it does look a lot nicer. I'm just not sure you really fixed some of the errors though. For one his abdominal muscles are much too small as they near the bottom of his torso and the obliques have been placed in the wrong spot. Also the position of his hand and fingers seems static and loose kinda like he's just letting it flop around while in the pose. When doing an activity like this most of the time even your hands will tense up. Try playing around with some cool "karate chop" hands or maybe something with the fingers spread, but whatever you do I'd like to see more tension an purposeful placement.
Picture Two: Yay, I'm not at work right now! /clicks. First thing that I noticed is that the perspective is off in the BG. If you're going to make the decision to tile the floor in the BG or draw a wood floor or a slatted ceiling, etc. . . you had damn well better get the perspective right because you've basically given your reader's eye guidelines right to your vanish point. Any errors here and they will be blaring ones. Make sure that desk fits the rest of the perspective you have chosen and that the tiles don't just abruptly skew to suit it's location.
As for the pants, well first off the rim isn't usually so big that it can be loose like that in this position. It would either be stretched tight or up higher. Also you should be able to see fabric from the back of his pants between his legs. The folds really aren't that bad I think you just need a little time to study clothing construction.
There's also an issue with his muscles seeming out of place or misshapen. More anatomy practice FTW.
Picture Three: You sick of me yet? Psh! Of course not! Okay okay okay, you really shouldn't draw every strand of the hair. It just doesn't look as nice or as deep or voluminous as when you block out the shapes and highlights first. Dark hair is drawn the same way I've just seen you draw blond hair.
Still seeing some issues with perspective here. The table, the window ledge and the blinds. They just don't line up like they should. Be careful with this.
Picture Four: My personal favorite ;D Not too many issues with this one, but you're right there is always room for improvement. Since you asked for help with clothes I'll give it. The way you've drawn them makes them look "lumpy" and at one point it even makes his left (our right) arm look broken! Oh noes!! Clothing drapes a figure and it will almost always hang and bunch at very predictable points. The only advice I can give for this is to learn the characteristics of different fabrics and draw from life. Learn how your clothes fold and pull and hang around your body. Look at them when you're just standing there and look at how they change when you move your limbs or twist your torso or hang upside down.
On a side-note: Yay, somebody who draws male nudity on this site besides me!! <3