OK, I looked at the code, and here's the story. On the settings page, font sizes 8,9,10,11,12,14,16 are available to choose, but you can actually type any value in and WinAGI will use it. My thinking was I didn't want to limit users if they really wanted to choose crazy settings.That value is then stored in the registry for next time WinAGI opens.
When WinAGI starts, it looks for settings (which should be in registry). If it can't find them, (such as first time it runs) it loads defaults. For font size, the default is 10pt. If it does find settings in registry (anytime after first run) all settings get a validation check (in case registry got corrupted or something else went wrong). For font size, if a setting found in registry is less than 4pt, WinAGI uses 4pt, and if greater than 72pt, it uses 72pt.
It sounds like something happened to the registry settings; it read something so it didn't use default of 10pt, but what it found was less than 4pt so it limited it to 4pt.
I'm thinking of reducing the band of acceptable font sizes to 8-16 and not letting users pick sizes outside that band. I really can't see anybody wanting to intentionally choose something outside that. That would at least keep things readable in case registry has a problem.
Assuming you didn't intentionally set the size to a low number, I'm not sure why WinAGI couldn't read your registry settings correctly. If you close and re-open, does it go back to 4pt, or does it now remember correctly what value you chose?