Visually speaking, it's best to use a combination of bulbs that produces an overall color you like in the tank. With only 4 bulbs of T5s, you will have less ability to refine that to an exact color you prefer, so I typically recommend staying slightly bluer (versus whiter) on your T5 bulb selection. You can then use your Kessil color control to add in additional white to get the exact color you like. Going the other direction doesn't work very well, i.e. if you have T5 bulbs that are too white, you can't really tune the color to make it bluer looking with your LEDs. The white T5s tend to dominate too much.
Now if you're adding T5s because you want to maximize growth and get full spectrum and don't care as much about visual appeal, consider a couple of ATI blue plus bulbs (or pure actinic) paired with 2 full spectrum bulbs such as 6500K, 10K, or ATI aquablue special. Those are very white bulbs, so if you want a less extreme white bulb, the ATI coral plus bulb is a good alternative that is still full spectrum but less so.
If you tell me what color % you are running on your kessils and prefer, I could give you a better idea of a more specific T5 bulb combination to closely match what you like.
Here are some popular 4 ATI bulb setups in order of deep blue to slightly blue:
- 2 blue plus and 2 actinic
- 3 blue plus and 1 actinic
- 4 blue plus
- 2 blue plus, 1 actinic, 1 coral plus (or purple plus)
- 3 blue plus and 1 coral plus (or purple plus)
- 2 blue plus and 2 coral plus
- 2 blue plus (or actinic) and 2 6500k / 10k type bulbs --> very white
