Trying to understand the problem, why would TRIM support matter if you're using 4 WD HDs in RAID 10? Trim is only for SSDs.
Are you saying you're planning on putting the SSD in the RAID 10 with the 4 HDs? Generally RAID 10 (or 01) is set up with an even number of equally sized disks. There really isn't a good way to incorporate your SSD into a RAID without buying another SSD.