In your list you say "small and portable Scheme implementation with AOT compilation and 4K footprint". However the presence of an AOT compiler is not the most interesting part. The distinguishing feature is that in 4K there is a REPL that uses a dynamic compiler to do the "eval". In other words, the code entered at the REPL runs essentially as fast as if it was compiled by the AOT compiler.

It seems to me (granted I haven't gotten it to work reliably at all yet) that the REPL is somewhat fragile, and I wonder if it might be easier as well as smaller to use a remote (tethered) compiler, as some Forth systems do.

Re the repl, I may have figured out what went wrong with user-defined functions. Instead of saying (define (sq x) (* x x)) I apparently have to say (define sq (lambda (x) (* x x))). Maybe that is documented in the paper, which I still have to read.