First impressions: There are many ways to increase score but not multiplier, and this seems to become a bottleneck. A smallish board and only 4 turns by default means you don't usually get to build up much or set up hooks etc. for bigger plays. When I upgraded random spaces on the board they were on the outer perimeter, which doesn't help much with only 4 turns.
The current format is a score chaser with a rising target a la Luck Be A Landlord, but personally I would prefer a bag-builder with a journey toward a defined end (i.e. not just survive as long as you can). I also think there's room for more interesting abilities than just xyz ways of increasing word score for what you use in the word. More control over what you draw, duplicating a rack tile or making it wild, removing tiles from the board, ignoring bad adjacencies (i.e. inter-space walls), stuff like that. There could also be different characters with cooldown abilities that could be upgraded or tech-treed over the course of a run.
Starting with a basic set of letters and buying more specialized tiles as you progress would also be more interesting to me- not just for different letter distributions per game, but also accruing tiles that have effects and aren't just letters.
Overall I enjoyed my first play and see lots of potential in the idea, but I'm not sure how long the current demo will hold my interest.
Edit: Also, since this essentially mimics Scrabble placement rules, something to keep in mind is that it's currently susceptible to the same sort of min-maxing by e.g. abusing two-letter combinations and rare words, so there's perhaps an open question of how much you want to incentivize that should you develop the game into something bigger.
