Hi Deb,
Just be carefull with the term crossover Different crossovers can be wave/free_style_ freeride/free_style_ wave/freeride Uncle Tom Cobbly and all!
If you are looking for a second sail with you first being 4.5 think how much you've improved since you started what do you weigh ( personal I know but essentially the bigger you are the larger the sail you can handle easily My friend Dawn is about 7st dripping wet can handle a 6.5 easily. ) and what you budget is. You're going to have catapults if you are just getting to grips with planing and so something with a bit of reinforcement isn't a bad idea. Kahuna has suggested the _expression_ but also in the NP range look at the Excess. What about a 5.5 Storm pretty tough and you already know how they sail. Check the usual suspects Robin Hood Boardwise etc. and ebay where you can get some good bargains, ( Windsurfers are generally pretty honest about gear ) and check out www.boards.co.uk for some of their sail reviews. At the end of the day the only way you will ever find out if a sail suits you is by riding it over a period of time, some you win some you lose, but have fun whilst you're doing it. Good Luck