So, I finished playing this level pack on hard difficulty a few days ago. It's quite clear that it was intended to bring new life to Keen 4. For one other example I recall, Keen 9.5: The Eight Accumulators also did this, albeit in different manners.
Despite the presence of the ruins of Sand Yego, as well as the appearance of the Pyramid of the Moons in the world map, the geographical connection to the Shadowlands as shown in Keen 4 was unexpectedly less clear to me at first. Maybe the location of the BwB confused me.
I eventually compared the two world maps, right after completing the level pack, thus clearing things up. Of course, it was only after figuring out how may they connect together that I already found an extended world map in the level pack's "extras" archive! I'm now wondering if I'm the only one who thought about the idea (just an idea, no execution) of combining Keen 4 and Return to the Shadowlands into a single level pack / mod; Possibly with modifications of original Keen 4 levels and/or more uses of RttS' soundtrack.
Keen 4 is known to have just a few music tracks. As expected, there was no change here, and with the possible exception of Sunken Sinkhole's music, you can actually make a 1-1 mapping of tracks between Keen 4 and RttS. Ignoring the re-use of "In a Land of Wonderment and Awe", these mappings should be clear:
"Shadows Don't Scare Commander Keen!!" -> "Returning To The Shadowlands"
"Welcome to a Kick In Yore Pants In Good Ole Hillville!" -> "Outskirts of a Good Time"
"You've Got to Eat Your Vegetables!!" -> "Keen 4 Song For Ceilick"
In particular, Dunesville, despite occurring in an environment with dunes and less vegetation, is an obvious take on Border Village, which also makes use of the matching music track. It makes sense that Soul Monster wrote that the level felt like a lost id stage, given that there was originally a plan to have a desert town in Keen 4. In fact, it looks like the unused world map desert town graphics were eventually used (with a modification) in this level pack! https://keenwiki.shikadi.net/wiki/Keen: ... esert_town
For the remaining tracks, the following mappings make the most sense to me:
"Too Hot to Handle" -> "Slow Roasted (Not Too Hot)"
"Tropical Ghost Oasis" -> "Cul-de-Slack"
There are still a few major differences in the ways the tracks are used. "Too Hot" is the most commonly used track in Keen 4; Not as much as in Catacomb 3-D, but it's used in a total of 8 levels, plus the scrolling story. "Slow Roasted" is similarly used in RttS' scrolling story, but when it comes to the levels, it's mostly used in the pyramid levels, and more generally, in levels surrounding the ancient. "Cul-de-Slack", on the other hand, is significantly more commonly used than "Oasis". Other tracks are also used at times when you'd expect a non-matching track, given earlier experience with Keen 4. This was obviously made on purpose.
I also got caught unprepared for the badoing in Sand Yego (and other levels)! I suppose it's somewhat close to Keen 6's Ceilick, only that it pops up from the floor, can be harder to spot, and there are a few more differences.
The Stingaru was another surprise when I stumbled upon it for the first time, in Jungri-la. So was the re-use of Keen 6's water graphics in the same level. I did identify the hut graphics from id's cancelled Keen 7 prototype, too.
Getting to the secret level was indeed tricky, albeit the relevant level has a clear hint. Of course, after additionally taking into account a variation of a hint originally present in Keen 4, I figured out the way. Truly, as someone else mentioned in the PCKF Discord, initially, I mistakenly assumed the secret level to be in a different location. In my case, this was the Pyramid of the Moons. Nevertheless, it's clear that the secret level gave a new theoretical background and perspective, applying also to Keen 4.
To finish, bugs are expected. One example is the name of "Pyramid of Footsteps" as shown while the level is loading. I also had one instance in which the game froze while in Sunken Sinkhole; I realize that it can probably not be reliably reproduced, and my guess is that it was related to two dopefish which were more-or-less overlapping beforehand. There were also possible minor bugs with tiling, albeit I'm unsure how many of these were intentional. I can send screenshots in some way, say via Discord.
Regardless, other potential players who are familiar with Keen 4-6 can give RttS a try for sure!