Nice puzzle game with cool concept! I haven't seen a lot of games with the mechanics of Four-in-a-row game so it was an interesting experiment.
The game was mostly easy to understand and play. Controls felt also natural. Although now thinking back I starting to was there a reason why you decided to use space instead of down arrow for dropping(?)
The in-game tutorial / instructions looks like really useful. I didn't unfortunately notice them in my attempt because the whole game did not fit into my screen and I just looked at the grid... I was going to comment that it was confusing to not know when another row spawns but then I opened the game again and noticed that it was actually very clearly explained!
The decision in the game felt meaningful for the first half but when more pieces started to pile up it felt like that I had no or only one options for good moves so it started to feel restricting. Also I noticed that screen filling increased the random elements of the game a lot which in my opinion lead the game a little bit away from the puzzle aspects. Sometimes the new pieces messed up plans that I had been building multiple rounds which was frustrating and sometimes the new pieces by random chances created new rows that actually ended up removing more pieces than was added which felt undeserved. I did not feel like I could control what happens when new rows are added and especially chaotic was when there was pieces added to only some rows and not all which messed up all the horizontal and diagonal rows that I had prepared. At first I thought I would lose if even one of the rows would be filled over and afterwards I think I would have preferred losing at that point because everything after that felt too chaotic. However I only played the game once so it might be that there is actually strategies that allow planning around the knowledge that the new pieces are added every 8 round. Additionally it should be noted that even if I do not personally enjoy puzzle games having a lot randomness involved many successful casual puzzles have a lot random elements without it bothering the players.
If you would add some sound and visual effects for polishing a game like this could work very well for a mobile game. It kind of reminds of match-three games because of the grid with gravity and the goal being to get similar elements aligned but it also has something to stand out from them.
If you have a chance to try either of the games I participated creating (DNA & Travelling Neighbors), please consider leaving at least some feedback for it. Positive or constructive are both welcome and greatly appreciated.