I've just knitted a rectangle...

[IronMonger]

...if I knit three more, can I make a jumper for DS?
Has anyone got a pattern or a way of finishing that they could enlighten me with?

Probably, but they would have to be slightly different in size, that is two the same for back and front and two slightly narrower and longer for the sleeves.
Have a look on Ravelry to see if you can find something called a 'rectangle jumper'