The all new Zebtoonz website!

[steve.ledger]

Take all the time you need, Steve! I'm thankful for your expert advice!

I've named the layer 1/2/3/4 respectively just for brevity, but you can name them uniquely as well.
So long as the suffix (lock) is added.

Each thumbnail link syntax then becomes:

popup: "1(lock)", close "2(lock)", "3(lock)", "4(lock)" (opens layer 1 and closes any other which were opened)

popup: "2(lock)", close "1(lock)", "3(lock)", "4(lock)" (opens layer 2 and closes any other which were opened)

popup: "3(lock)", close "2(lock)", "1(lock)", "4(lock)" (opens layer 3 and closes any other which were opened)

popup: "4(lock)", close "2(lock)", "3(lock)", "1(lock)" (opens layer 4 and closes any other which were opened)

No close buttons necessary.
See attached for example.