Pine is very tricky for "real" furniture. If your goal is something fully functional, sturdy, but with the general appearance of a dorm loft, pine will work great. If you are going for something a bit more refined, pine is tough to work with. It loves to warp and twist a little until it is FULLY dry, and none of the pine lumber you can buy at a home center is likely to be fully dry. Also, getting all the knots fully smoothed out if you are using pine for the tabletop takes some practice.

Basically, you can save money on the lumber by going with pine, but it is a lot more fiddly to work with. And if you like a darker look to the wood when you are done, staining pine is VERY fiddly.

Four legs is plenty to hold the weight, but you may want a couple of stretches running on the underside of the tabletop to keep it from bowing.

I'd give some thought to just making the top out of plywood. It's already flat and stable. You can get some iron-on edging to trim the edges so it doesn't look like plywood.

Standard desk height is about 29 inches. I'd shoot for that unless you are particularly tall or short.

What tools do you have to work with? That will have a big effect on what you can pull off.

Here is the computer desk I built out of pine. So I know of what I speak when I say working with pine is fiddly.