LaCie 4TB stuck on Erasing HD in Time Machine

I don’t know what to do. I’m scared if I force quit or restart my computer I will ruin the external HD or something.

It shouldn't... but the symptoms you describe suggest it may have already failed. Erasing a disk should only take a moment or two: Erase and reformat a storage device in Disk Utility on Mac - Apple Support

If Disk Utility is stuck and unresponsive, force-quitting the app is justified. Restarting the Mac will accomplish the same thing, but Disk Utility's apparent unresponsiveness will probably prevent the Mac from shutting down anyway. So, you might as well force ** to quit: How to force an app to quit on your Mac - Apple Support

Even if it were to "ruin" your external drive, you still have the data on your Mac.

It'd be a bit of a hassle, but drives fail all the time and the fix is to get a new one and backup immediately. The chances of you losing data during the time interval of having no backup is remote if your Mac is operating normally.

As it stands, you don't have a backup anyway ... what's to lose? Inaccessible backup data ... ???