Hey Speedcube - first off thanks so much for checking out our demo! I appreciate you taking the time to do a speedrun and write up some feedback. I'll be honest - the '4' thing is just a coincidence! However I really like that idea as something to possibly integrate into the main game somehow.
Second, your theory about the score breakdown is essentially spot on. You have to finish the run in 4 or less minutes and get at least a 40k score for the Legend rank. Downing enemies and the boss gives a little bit of a boost too, as well as collecting any items, blasting Skully, and finishing with as little damage as possible. We plan to factor even more variables into the final game once we get there, as well as subtracting from your score if you do or don't do certain things (use too many medkits, reload too many times, time bonuses, etc.)
Finally, I'm curious what might've gotten more difficult in the versions after 5.3. The only things we've implemented since then were the flashlights and minor bug fixes not relating to anything gameplay-wise, so I'll have to do some investigating there.
Thanks again for your post!