The 4 to 5 degree gap is just a generic response. It generally applies to a set of irons or hybrids but when going from a driver to a fairway to a hybrid to a utility to your irons there is no set rule concerning the degrees. They all have their own characteristics and with the vast array of shaft availability the best answer is a full bag fitting or a lot of trial and error.

I’m a 7 handicap and here is my current gapping:

Driver 10.5
3wd 15.5
3 hybrid 21
4 iron 24

My suggestion would be to focus on how far you hit each club and the trajectory and not so much the degree of loft. I carry a 23 degree hybrid and a 6 iron that I hit similar distances but the hybrid has a much higher trajectory. I do not carry any irons above the 6 iron and instead use the hybrid, a five wood, and a three wood. For me, I find it more important to fill gaps in shorter clubs and feel comfortable I can cover any shots between my 3 wood down. I know my set up is different from other folks and that's okay with me. Go with what works for you.

There's a decent chance that 21* hybrid goes farther than your 20* 4 iron, so that could already be a good gap. I guess it depends how many wedges you need (probably a fair amount since those t300 are pretty strong lofts.) I personally go 10* driver, 14.5* 3w, 17* hybrid, 21* hybrid, 25* 5 iron. First spend some time with that 21* hybrid and your 4 iron and see what the distance difference is and go from there. If you have room for another club between 3w and that hybrid I'd suggest a 18* 5w or hybrid.

I second what Dale said. You really should focus on not only how far you hit each club and fill with any gaps should you have them, but also how you use each club. Hybrid will most likely have a higher ball flight and land softer than a long iron or driving iron.
I go Driver 9*, 3 wood 15* then to T200 3 iron 20* (utility build) then to my 4 iron. For my game and how I play the 3 iron is my go to club on shorter par 4's and fills the gap between 3 wood and 4 iron perfectly.
I prefer to have more options at the lower end of my bag with wedges.

Do you know your distances with your 6-5-4 irons? If your 4 iron isn’t at least 12-15 yards longer than your 5 iron, but your 21 hybrid is, then use the 4 iron to clip weeds in your yard. You can then find another metal wood to fit in between the 21 hybrid and the 15 wood. The obvious starting point would be an 18 degree fairway but get the numbers based on your swing, distance, and flight.
Knowing your distances and having clubs to hit common distances is more important than having X degrees between each. With the T300 P at 43 degrees, would you need another wedge to cover all distances from 140 yards and in?
The possibilities are endless. If you can hit a 4 iron, then maybe a 2 or 3 driving iron instead of your hybrid would work. Seek help worth what you pay for. You pay nothing for our advice on equipment.

What you see is my entire arsenal.

Wood gapping for my complete bag is the 10.5 driver (set to 12*)-17F-21F-27H, 7-P (48), 56-64. Very little difference between 23 and 27H and 6-7 irons.

Sunday bag is 12 driver (set to 13.5) -21F-27H, 7 & 9 irons, 48-56-64 wedges.