4 layers is rather easy to conceptualize: You just have an extra cell line grow out of the ectoderm as well as the endoderm. Or even you could have 2 line branch from the endoderm, or even 2 from the ectoderm
There’s no reason why diploblasts couldn’t become more complex. Sure, complex Earth animals have an extra layer, but I don’t see why that is necessary: You can make a lot of crazy organs with just the ectoderm and endoderm, both in diploblasts and triploblasts. And if mesoglea is an issue, just get rid of it and have the two layers stick together