It's not limited to 4K, it has something to do with dpi scaling. I can experience this issue on 1080p, it's simply much worse on 4K.

This laptop is 13" 1080p with dpi scaling at 125%. If I remote desktop to my other laptop (1366x768 100% dpi) the problem occurs, but only after mIRC is restarted while remote desktop is active - there's a Windows setting to apply scaling based on log-in resolution vs resolution at program start. The weird part is the exact same script looks fine running natively on the 1080p laptop. Though I don't blame mIRC for this really, Windows has terrible scaling support in general. Its own tools don't even scale properly.