I'm a bit of a veteran when it comes to this game, having played back when it was Pokémon Crater 12 years ago or whenever that was around. There are two things in particular which are increasingly frustrating right now and very off putting for the game.
Selecting moves - This doesn't always register. I'm posting this because I just lost out on a Mystic Shaymin Sky as a result of this. I selected Solarbeam to be used from my Dunsparce which being x4 not very effective would have not killed it. However, it selected my top move of Trump Card instead and knocked it out. Incredibly annoying, not sure why it happens. But this has happened a lot since V4 came around and occurs with item selection too. (I know this is a bug but I wanted to combine it with an actual bit of feedback/suggestion.)
Evolving - It used to be so easy to know what to evolve. Having the whole Pokédex on a list meant you could scroll the whole way down and see which ones you need. Now you have to just know in advance what you need in order to check. I don't have the motivation to cycle through 70 pages of Pokémon. It's incredibly tedious and is massively decreasing my play time as I'd love to get back into collecting but this is impossible under the current system.
Otherwise, wanted to say good work on V4 so far. Although it's a little too close to Facebook for my liking, I understand the reasoning behind that. It's very clean looking and ultimately has been optimised which is good to see as it finally looks like a professional site - coming a long way from when I started. Hope this gets read and I apologise if it's been brought up before.