The PrefixAllGlobals sniff show that 3 letters are the minimum to make prefixes for functions and variables, but in the Plugin Review Team, We're using a minimum of 4.

We ask developers to use these minimum letters and this is our argument:

Don't try to use two (2) or three (3) letter prefixes anymore. We host nearly 100-thousand plugins on WordPress.org alone. There are tens of thousands more outside our servers. Believe us, you’re going to run into conflicts.

So, let's update this sniff.

It solves updating the number to 4 in line of the sniff.

Related with #1733 and and the update of Developer documentation.