I've played this game several times but have returned after a lapse of about a year and I am receiving a runtime error message (attached) and don't know how to get past this. Any help would be gratefully received.

This seems to be an error known for a long time (I found a post from someone asking the same question back in 2011).
However: here's a solution (admittedly for the Steam version, but hey...beggars can't be choosers), that hopefully will work for you, too:

https://steamcommunity.com/app/517810/discussions/0/350542145688029383/?l=english

(I don't own Caesar 4 myself on either GOG or Steam, so I can't confirm the efficiacy of this solution - but I wiish you luck)

The repair solution that worked for my Windows 11 was given by BREOL72 - I will repeat it here for clarity:
- navigate to the directory of Caesar IV installation (D:\GOG Games\Caesar IV)
- right-click on the file RegisterDLL.bat
- click Run as Administrator - Command window will show up for a second
- right click on Caesar IV.exe and "Run as Administrator"
The game should start OK
Subsequent starts of the game I did from the usual Desktop shortcuts.
HTH