With the Fae, and soon Orc and others, being introduced it has opened to door to get additional character slots.
The new VIP pay to play is what inspired me to suggest another option, since there is an obvious mechanic in play to expand the 4 slot max and also an obvious need for additional funding to produce the game.

A simple one time fee for a single extra character.
For many who have been playing forever, the current slots are fully utilized (for storage if nothing else). Giving one up to examine a new race's gameplay is not an option.

If that is a deal breaker, then possibly ponder door #2 . The ability to change the race of an existing character.
Converting a mule character into an Orc, or Fae, or whatever.