Sky subscription won’t let you just get SS2. I think it’s either 1+2 or 1,2,3+4, then HD. Virgin’s no doubt different.
The only division is that SS4 shows the less mainstream stuff. Most live events are on 1 or 2 (not a particular division between what is shown on each), then repeated on 3 and sometimes 4.
So you won’t miss much with a subscription that gets you just 1+2.
Rugby-wise (couldn’t tell you about cricket or golf), without SS3, you’d miss some of the Super Rugby, but not much else.
Hope that helps!
(I think you can start it and cancel without notice, so you could switch on for a month, then cancel it)