For the pi-4 which should i use Ubuntu Mate or LinuxMint for desktop use

If you are using RPi4 then install UBUNTU LTS 20.04 and configure the wifi. Then install MATE-Desktop. UBUNTU is a perfect choice for RPi4. For browsing you can use firefox. And you have the snap store on this OS too.

Ive been using ubuntu on pi3b for longtime , smooth for what hardware will allow , image its pretty solid on pi4

Year 2020. Ubuntu actual 64-bit OS, not like the raspbian, which is still only 32 bit. Download the server then you have three desktops you can choose to use in GUI. https://ubuntu.com/download/raspberry-pi

Even the 4GB Pi 4 Model B uses an ARM based chipset (CPU) which (to my knowledge) Linux Mint doesn't support (and I believe that includes Ubuntu, considering they're both closely related).

I don't know if it has to do with the Kernel or the desktop environment. I honestly THOUGHT the last Kernel update included ARM support, but ... However, (at least at the time of this entry) you do have nearly a dozen (all Debian based?) alternatives (at least ONE of which might be versatile enough to customize an allllmost Ubuntu or Mint-like feel). Best you can do for now... [Shrugs] Either that, or you could pick up a used laptop (or even an older quad core DESKTOP, if you don't need the mobility) for around the same price as a Pi 4 after a cooler and case, etc and Mint/Ubuntu would likely run fine, of course.