Why not start now? But the answer depends on how much you can practice.
It's my belief that the sooner you start these things the better - not only have you got more time to get the fine detail right but you can devote more time to other exam things like aural and sightreading.
If you progress well and find that you know the pieces by, say, January, play other pieces for for a while then go back to them refreshed later on. Use any spare time to practice sight reading. Why not have a go at sightreading the old grade 2 pieces you didn't play? If they're hard, sightread just the melodies. Doesn't matter how much you stumble it's all practice you'll be glad of later on!

Alternatively you can apply a little theory - learn about cadences and try to recognise the ones in your pieces. Write out the scales (treble clef) of the keys the pieces are in. It can only help with appreciation of music because you have to listen. (And it'll put you that little bit ahead for your grade 5 theory when you get there. You'll have less to remember because you'll know and understand some of the stuff.)
Good luck