Hi @frogfan44 - It does sound like the 4k display is probably the "culprit" (or TS4 is the culprit, and your 4k display is just awesome 😃)
Try this: right-click the Sims 4 icon on your desktop, at the bottom of the Shortcut tab click on "open file location". This should bring up Windows Explorer and highlight the TS4.exe file.
Right-click the highlighted file > choose Properties > Compatibility tab > Make sure "Run this program as Administrator" is checked > Click "Change High DPI Settings" > At the very bottom, check the box "Override high dpi scaling" and choose "Scaling performed by application". Click "OK" to exit and "OK" through all the windows until you're back to the file.
Then launch TS4 and see if it looks better.
I don't have a lot of experience with TS4, but I know in SimCity 2013 the game does not support 4k. This works well in that game for 4k display. It's a much older game, with no active support, so there might be better support for 4k in TS4, I don't know. (Also, just so you know, the default location for the options.ini file is in your documents folder - \"YourWindowsUserName"\Documents\Electronic Arts\The Sims 4
Please let us know if this works for you, so others with the same problem can find it. If it doesn't work, there's probably someone around here that can better help you. Happy Simming!