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.
Worse on 4K because more likely that you will need scaling > 175% which is custom scaling. As per my previous post, scaling using dropdown values of 125%, 150% or 175% seems to be OK.
This laptop is 13" 1080p with dpi scaling at 125%.
You need to specify:
a. Windows version i.e. Windows 10 1709
b. Whether it is 125% using the dropdown or custom scaling or old-style control panel.
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.
Aaarrrrrrrrgggggggggghhhhhhhhhhhhhhh!!!!!!!!!!!!!!!!!!!!! Please let's not complicate this at the moment by introducing issues that occur when you are using remote desktop, which has scaling settings at both ends plus variations on remote desktop software. Perhaps we can fix the native issues first, and then look at remote desktop as a second phase.
Though I don't blame mIRC for this really, Windows has terrible scaling support in general. Its own tools don't even scale properly.https://i.imgur.com/OaHe7XN.png
This image is not an example of Windows own tools not scaling properly.