Q:

Why the first user here has number 4, not 1?

Just out of curiosity.

A search for User#1 returns nothing.
A search for User#2 returns nothing.
A search for User#3 returns nothing.
Finally a search for User#4 shows FDSg.

Searches for users up to #10 do return results, except one.
Why is that so? WHY THE GAPS?
I suspect one of the following:

the first users are SE employees and admins, so they have no posts (as they are just running the site) and thus are noth found by the search engine; am I right?
the first users had their accounts deleted. Less likely, as AFAIK people insist on deleting their accounts only in rare situations, and I can't see how could the very first users be vandals/spammers.

A:

Some user accounts have been either merged or deleted
See also The number of users - curious jumps

Other than that use the URL template https://mathematica.stackexchange.com/users/# where # is the user number, if looking for users by number.
You will find:

Geoff Dalgas
Nick Craver
Rebecca Chernoff

The first three accounts were created for Stack Exchange employees to administer the site, especially during the beta phase.  You were correct.

