I think it's great that you are trying to extend yourself to harder pieces, however you don't have to go too overboard. Aside from your grade 4 exam pieces, I would say start with some easier pieces which you feel confident in playing, and then work your way up once you've mastered each piece. Honestly, you can assess your own ability more than me, so just follow what feels right for you. Also, ask your piano teacher for advice and what he/she thinks.
There are obviously always going to be some pieces beyond your ability, but don't worry that. Eventually your playing will improve, it's only a matter of time if you are consistent and committed with your practice. Also, make sure you make a good solid foundation before you try a very challenging piece, don't jump straight into the deep end so soon. Don't neglect your scales and music theory (like I did). Scale fingerings are the basis for most practical and functional fingering choices with scale-like passages in most repertoires. Once you master all of them, it's gets a lot easier to play various parts of many pieces. If you get stuck on a very technically challenging part on, try some of the Hanon's exercises that can warm you up and help with whatever you are trying to play
http://www.freepdfsheetmusic.com/han...ar=H&FileType=
And last and not least, try not to get too caught with the technical side. Musicality, expression and stylistic sense is just as important and don't forget to play with feeling