Wow! I've done it!
At a loss to think of anything else, I clicked rapidly on the '4' at the top of page 3 - perhaps a dozen times - and, lo and behold, Page 4 appeared.
I went back to the GD index page and clicked the '4' of the thread (just once) and Page 4 opened again. Fixed it... I think.
Well, I hope that's a possible work-around should anything similar happen again.
Now, if anyone wishes to show their gratitude for this helpful information, please PM me for the address of 'The make JB a rich man' charity foundation.