4 users in 1 row? Not possible. Each user is a database table row. If you mean you want a bottom border or something styled after the 4th row instead of each then you'd need to consult with your template developer. The example below would do the trick, but I've no idea if you have other CSS present that'd conflict.

Please note the above is a CSS3 selector and only works on modern browsers (IE8, IE9, Chrome, FF, etc..).

Well it's not possible depending on what you're wanting. I don't understand your request fully. If you have 4 columns and each column has fields then of course not, but if you want 4 users displayed across 4 columns (1 column per user) then I guess it can be done. Could you provide a screenshot, picture, or something of what you're wanting so visually can understand your needs. In nearly all cases all visual changes can be done with CSS avoiding core edits or new development.