Literature and History can be really hard if you don't put the effort in
Gov + Pol is really easy, especially as it's still currently modular (idk about later years tho)
I wouldn't know much about politics, but with Law at uni, you don't have to have certain A-levels to take it, so just do what you want to do
Also... bit of a side note but I know that Oxford (and therefore most likely other uni's) don't care too much about a 4th A level because not all colleges give the option to carry on to their students... stuff about it being unfair blah blah blah