As we start to close out another year, where are we at with K4 and what have we learned?

I’ll start:

My recurring idea for K4 is a superencipherment (adding or subtracting a keyword mod26) on top of a substitution cipher.  I can think of keywords and different ideas for likely substitution methods but have so far had no luck.  Some times I’m not sure I’d know what to do with the primary ciphertext once I’d stripped the masking technique.

So what other ideas are out there?  Try and break it down to a simple sentence or two.  What do we think are the most likely candidates for possible methods used in K4?  We’re not trying to steal your secrets, keep it general but definitely share.