Whether you should get this or not is quite simple. If you played and liked the previous Deponia games, then you're likely to enjoy this one as well, so buy it. If you didn't play them, then start with the first Deponia, followed up by parts two, three and four depending on whether you enjoyed it enough to keep playing (the second game is particularly strong). If you didn't like them, then this fourth part isn't going to change your mind, so don't buy it.
Play it in German if you can. Otherwise use German voices with subtitles in your preferred language, as the German voices are much more fitting than the English ones and are a big part of the comedic timing in scenes.
I really liked the first three games and I really wanted to like this one as well, but I couldn't. It starts with some of the new mechanics. I really dislike beeing stressed by a time limit in adventures. Some of the old ones (e.g. Broken Sword) pulled those limits off quite well, but there you would usually only have to replay one decision should you fail. Here the loops are bigger, too big for my taste and that annoyed me. The next thing that annoyed me were the puzzles. I am a veteran concerning adventures and I have seen a lot of different adventure styles over the years. I am usually quite good at finding out what to do, but here I had my problems. It is normal for some puzzles to stretch logic a little, especially in comic-style adventures, but in this game, there were too many of those. Coupled with the time limits, it sometimes felt more of a chore than a game. The humor from the first three games also seemed to be missing. The first three games made me chuckle quite frequently, this one didn't. There were some funny scenes, but not that many overall, or at least not many that I actually found funny. Another thing I was missing was the singer between acts. He appeared at the beginning and I can't remember if he showed up again later. He was a constant in the first three games, why was he missing here? But my biggest problem with the whole game was the story. It doesn't add anything to the story told in games one to three. The plot was either predictable or seemed random, not much in between. So all in all, this game seemed quite unnecessary to me.
Unlike its predecessors, thanks to QTEs this adventure game is now unplayable for people with disabilities that cannot provide rapid input in a very short amount of time.
I thoroughly enjoyed the previous Deponia games but the addition of QTEs make the game literally unplayable for me.
I guess there is no discussion that the Deponia series had a very positive influence on the development and perception of point'n'click adventures. Apart from its somewhat unique setting, the effort (and money) that was invested in animation and music is unrivaled at least in the last decade.
Then again, I must admit that I never totally loved the games. The protagonist was not very likable, the puzzles were somewhat erratic and the mini games were mostly annoying and technically bad. And, well, there was this ending that most people didn't like and that the developers defended to the death. My personal opinion was that it didn't fit the game but mainly felt rushed and unfinished.
To sum it up: the puzzle style is unchanged (only junk in inventory used in nonsensical ways), the chapter songs were removed (apart from a crappy one at the beginning and an unrelated song later), the mini games are still total crap (technically and in any other way) and the end will create more or less the same mixed emotions as that of the original trilogy. It's a bit like the developers felt the urge to tell us that everybody who didn't like the original ending was a complete and utter idiot.
What I absolutely hated was the time pressure especially towards the end. I don't like time pressure in action games with well designed controls but it's an absolutely terrible idea in point'n'click adventures with sluggish controls. The whole time travel thing leads to entering the same scenes over and over again with only minimum changes (if at all) and at some point, the whole story is such a mess that you stop thinking about if there is still any sense in it at all.
So, well, I really don't know if this was worth it or if the developers should have let the trilogy as it was. IMHO it would have been better to simply add a better ending to part three. Not necessarily a different one, but one that didn't feel so unfinished.
The end of the third game was mostly hated, I liked it since for me it made sense with Rufus' development as a character but I understood why people hated it. However, this game is just the author of the game giving the middle finger to everyone that didn't like the ending of the last game plus boring puzzles and a more miss than hit humor.
I would avoid this game entirely, buy the trilogy and ignore this one as a real sequel in which you play as Goal would have been way better.