If I consider these images, 1-3 are decent, while in 4 the lightining is way harsh and looks too much like a P&S shoot.
However, trying to be constructive, I think that you could have managed much better with a bit more attention paid to setting or pre shoot arrangements. After all, in formal protraits like these where you can arrange the setting to meet your needs, it typically pays of to consider the setting for a minute.
In my opinion number 3 here is the best when the setting is considered, while I think that number 1 could have have been very good if the setting was bit more considered.
You see, the reason I prefer number 3 is the more isolated background when compared to other shots. And the reason for this is not your photographic technique (I checked the settings from your web site), but the arrangements. The only probelem here is that the model is sitting too near to the background which makes isolating background much harder. Although in number 3, where he leans just a bit nearer, the background is way more isolated than in other images. Which shows that 50 cm or two feet of distance is this respect has significant effect on the final image.
In this respect, next time you try portraits, you should consider if you can increase the distance between the subject and the background to get something looking more like number 3 here. In fact, I would say that if the background of number 3 was in number 1, it would have been very nice protrait (with slight crop, you could have made a nice contrast for the vertical lines of background and horisontals on the shirt).
Hope that this helps.