Hey, most of the online versions of the game I've seen use a 10% spawn rate for 4s, this is verifiable from integrated counters in some of the websites and people who know more about computers than me looking into the backend. I've been using the iOS app casually and (I'm a numbers person) so I've been curious about the spawn rate, and I was wondering if I would be able to estimate it using some of the saved games, the app saves your 5 most recent high scores (not sure if introducing bias by using games I've done well on) and it turns out it's not too hard (using the count of each tile, you basically can figure out what the score would have been if you'd only generated 4s then you can figure out how many 4s got formed by 2s and you got points for which gives you the total number of 2s spawned and you can work out the total number of 4s and get the percent of 4s formed in that game and from that estimate the actual rate) and it seems like the rate is closer to 15%. This is based on almost 20,000 tile spawns across 5 games.

It is unclear if this is significant in any manor, the only way I can think of is someone using a bluetooth keyboard to play on an a phone or tablet of some kind, since having more 4s spawn means you have to make fewer moves to build the same mass in the tiles, this would be a slight advantage. I haven't watched any video of records to see if anyone has done this, but if so, someone should watch to see the actual spawn rate. I'm not too involved in the thing for this since I am not very fast and I use touchscreen, but did this and then googled it to see if there was a consensus rate they spawned at and was surprised to see it was only 10% and figured someone should know.