Depending on the type of RAID controller you've got it might be possible to add more disks into the existing array, there are however a number of things to be aware of. RAID0 has no redundancy and hence one drive failure (or even a bad hiccup) will leave you dead in the water, the chance of this happening with 2 disks is quite low but with 4 disks in the array you've got a much higher chance of failure. If you do decide to expand the array then it won't expand the partitions which are on it, you'll need to use Partition Magic to expand the drives within windows to fill the available space on the array.
Personally I wouldn't put more than 2 disks in a RAID0 array, the speed difference of 4 over 2 isn't great enough in my mind to offset the risk of failure. If you need more space you'd be better off just adding in one or two more single disks and moving stuff which isn't speed critical onto them (photos, music etc).