I had issues with Pi4 wifi too. In my case I was noticing it when sending files over Samba from W10 or using FTP, and also scraping problems. I'd get constant disconnects and transfer speeds of around 200kb/s even when it would stay working.
I found mentions online of known issues where noise from the HDMI output can disrupt the wifi signal. Every instance I saw talked about said it was only over a certain resolution (1440 I think), but I decided to try disconnecting my 5" screen anyway, which runs 800x480, and as soon as I did that the speeds went up to around 3-5MB/s and the connection stays stable.