Looking at your motherboard manual, your machine has a total of 4x SATA3 ports, and 4x SATA2 ports. However, 2 of your SATA3 ports are provided by a different ASMedia chipset, and as such can't be added to your RAID.
Intel Z77 Express Chipset
2x SATA 6.0 Gb/s ports (gray) with RAID 0,
1, 5, 10 support
4x SATA 3.0 Gb/s ports (blue) with RAID 0, 1, 5, 10
support
So you could add two more SSDs but they would be limited to 3.0Gb/s, losing around 200MB/s+ off the max speed of an 850 EVO.
I'm not sure if the cost would justify the speed in this case, and the added risk of failure with a larger RAID, as when using RAID 0, should ANY drive in the RAID array fail, you will lose all of your data.
As pointed out by @Ramhound:
You would be better off migrating to RAID 5 if you plan to use 4 disks. That will give you both the speed advantages of RAID 0 and the ability to lose a disks ( RAID 1 ). But by using all your ports you will lose some of your speed performance but in my opionion the advantages of RAID 5 would overcome that.
This is, if you plan to upgrade to a dedicated RAID controller. This would give you the ability to rebuild your array without losing anything other than time. Note also that this is only viable for a single drive failure, should two drives die at the same time you would be in the same position.
EDIT for below point: Different sources are giving me different figures, so I can't confirm this math is correct.
If you really wanted to get the best storage speed, your best bet is a dedicated RAID card using 8x or 16x PCI-E 2.0, or a PCI-E 3.0 4x or above, with at least 4x SATA3 6.0Gb/s ports. The speed of 4 SSDs in RAID0 can be utilized, with the right hardware.