Jonathan,
You should be fine with it. Just compare it with your current body and see if the meter works normally, and that it sounds normal (maybe reattach the MB-21 for comparison purposes).
Electrical issues can occur with the late bodies as well. I had a 2506xxx serial body that was intermittent in function. Sometimes would just lock up and start angrily growling without doing anything. My earliest body was fine, except that the mode dial was a little tempermental. Sometimes would engage A or P mode while the dial was set to M. Both cameras were well-used. I had a third body that also had electrical issues. A 2434xxx body. Wouldn't fire a flash attached to the hot shoe, even if a different prism was used. The original prism was fried and didn't show the proper indications in the LCD.
What you could do is see if the seller would trade the MB-23 for your MB-21 and some cash. Then, you can have the MB-20 and the MB-23 for your current F4.
-J