I am no expert, just attempting something similar. In the stores it felt like the soft hybrids were softer but more supportive than the soft all latex, if that makes any sense. 4" of soft would probably be too soft for us and we liked the mediums of most of the hybrids in store.
Since dunlop of the same ILD can feel a little firmer to some people, maybe a middle layer of dunlop would be good, especially if you can find one with a little higher ILD like Sleep on Latex (20 ILD). It would give you a chance to switch around layers too for a different feel. Keep in mind I am getting caught up in the different ILD’s from the different stores to the point I can’t make a decision for myself.
Another option is if you are pretty sure you like most soft beds, you could start with 3" of soft talalay, see how you like it, then add on from there and get the cover last.
If we were going to do another foam build, I’d go with a 6" dunlop base, then a 3" medium layer, and a 2" or 3" soft layer.
Either way, good luck and post what you end up with.