I have just added a fourth user to my test forum. While the first three users have the IDs of 1, 2, 3 respectively, the fourth user has the ID of 10 instead of 5. Is this normal?

Perhaps this has something to do with the fact that upon a time I had a few more users on the test forum (they since been all remvoved though)?

Do user ID numbers adjust themselves to reflect changes after certain members have been deleted? For example, does Jack (ID#10) become ID#9 when Peter (current ID#9) is removed?

Thanks.

--MB

Do user ID numbers adjust themselves to reflect changes after certain members have been deleted? For example, does Jack (ID#10) become ID#9 when Peter (current ID#9) is removed?

Definitely not :)
ID has the proprety AUTO-INCREMENT in the DB: in fact, when you create a user, ID is incremented of 1 and affected to the last user registered. If the last user is deleted, ID value does'nt change.

fourth user has the ID of 10 instead of 5

That means that you created 10 users (the fourth users is in fact the user 10 ) but you deleted five users among this ten users.

Ah-hah, I see! Many thanks for your explanation, Cakeman.
Cheers,
MB