Google "wood thread insert" they hold in deeper than a tee nut, and work EXCELLENT. I have used the same style casters you have on a wood workbench before, and have never had an issue, they are very secure.
I would assume that the reason you cannot use a four bolt caster is due to space on the end of the table leg, the same thing would apply to tee nuts (although smaller than a four bolt caster), but the inserts take up barely more room than the caster's thread.