Line of Duty - Late to the party

[dalek]

Just finished series 1 and first two episodes of series 2 - how did i miss this first time around?

No spoilers please

There's one big thing that you'd know if you watched series 4 before series 3.
I definitely wouldn't
4 is supposed to be standalone but there's a lot of things that provide background.
And if you're going to watch 3 anyway I'd watch it in order.