Are your snap and pop keys shifts (works like shift), dead keys (I full press & release, then strike another key), or locks (like a dead key, but good until I strike it again to unlock it, e.g., caps lock)?

(Let's say lower case is your first layer, upper case is your second layer, snap is your third layer, and pop is your fourth layer.)

I've worked on a similar design, and I've replaced Caps Lock with a "level 2 shift", which gets me to third layer. (Perhaps in your case, "Snap") The fourth layer is then Level 2 Shift and normal Shift, together.

If your snap & pop keys are shifts, combined with normal shift, you have a total of eight layers, potentially.

(I was inspired by the Neo Layout[1], but it's for German.)

(Also, since programmers often use - and _ for word separators, I put those on the space (which is a word separator) bar; they're level3+space and level3+shift+space, though I don't remember which was which. This made sense to me, but keyboard layouts are a personal thing, I think.)

[1]: https://neo-layout.org/index_en.html