The forth thread is usually a seaming stitch. Actually I am not really sure because mine is a 5 thread. I have 3 needles and and 3 loopers. When my machine is set for 4 threads there is two straight stitches running down a wide-over lock stitch making it a suitable seam. Just 3 threads are used mainly for seam finishing and decoration.
When adjusted, the 3 thread will give you a rolled hem. Normally, a 3 thread is fine for finishing an edge . but is not as strong holding fabrics together. If you want to sew and finish at the same time, use the 4 thread. A 4 thread can give you a flat felled seam. You will use both in serging.
If you haven't bought yet, go for it lady.The difference is just the 4th thread. I owned a 3 thread
for over 20 years, it was a great serger, Babylock, I used it mainly to finish seams specially on fabric that ravels, great on hems, no loose threads. Gives garments a more profesional finish, excellent on knits.
just remember sew the first seam with sewing machine, try garment on then if you're happy the way it fits now you can finish the seams with the serger including the hem edge, no more straggly threads anywhere. There are a few books that
teach you how to sew entire garments with the serger, my fav are slacks or crops after cutting fabric youcan sew a pair in about 20-25 minutes. Enjoy!
The three thread serger makes a wonderful rolled hem but does not make as secure a seam as the 4 thread. Both make a very stretchy stitch but the 4 thread has an extra stitch line (a safety stitch) which makes it much more useful for serging the garment together. If all you want is to use a serger for clean finishing seam allowances, either machine will do. I have used both for many years and would always opt for the 4 thread.