Optimum configuration for 4 SSD drives
With spinning-platter disks you want to have the logs and data on separate drives as random access data disrupts the sequential log write operations, making the logs a performance bottleneck.  SSDs do not have this issue as they lack the performance constraints imposed by the mechanical action of conventional hard disks.

If you're getting SSDs for a DB server, get ones designed for a long life span, such as Intel S3700s.  You're probably better off getting two of those than four cheaper ones, and 2x100GB units + a hot spare (which is probably enough to keep you going for a few years at your current rate of growth) should cost around £200 each according to Google Shopping.  

Mixing logs and data on the same disks isn't such a big deal any more.  However, you may get some resiliency out of having separate log drives.  If you want to do this then 5 disks (enough for two pairs + a hot spare) should still see you with change out of £1,000.

Try two good quality drives in a RAID-1, with a hot spare for a starter.  Your growth suggests you probably won't have transaction volumes so high that this setup can't keep up with your application workload.

Back up your database onto spinning disks - a couple of enterprise-grade SATA disks and a hot spare should cost you a few hundred dollars.

If (and only if) you have performance problems with that lot then you might look into adding more SSDs, but I suspect just one pair will be fast enough to keep up with your transaction volumes quite comfortably.