It sounds like it's time to bite the bullet and start upgrading. The fact that your wall switch is wired in means it's sending powerline signals, not RF, which means many of the converters mentioned above are probably not applicable. Leviton has wifi switches and they have a very similar 4-rocker switch for them; Lutron has their Caseta line which works awesomely (same protocol as the RadioRA2 professional solution), or there's Z-Wave or UPB. UPB is kinda dead in my opinion (as an ex-user of it, this is disappointing)... Wifi may have device limits with all that noise on your wifi; z-wave works pretty well but may need some occasional maintenance - so there's no obvious solution (ok Maybe Caseta if you like the control pad options).
Time to do some research and pick a new protocol I think. I was a UPB user in the past, and while I really like it, the fact that they never kept up and bridged into modern consumer applications or integrated with Alexa or HomeKit is really disappointing. It worked really well for me otherwise. When I moved out of my house that had ~70 UPB switches, I moved to RadioRA2 and couldn't be happier except when it comes time to pay for it - it's quite expensive - but there's nothing better than laying in bed, watching TV on my AppleTV and just saying "Hey Siri, turn off the lights" and it does. Good luck in your research - and if you want to ask questions, enough of us here have experience with all the protocols. I've personally used Zigbee, Z-Wave, UPB, Hue, and RadioRA2.