So, a stab at a 4e...
MUL:
Ability Scores: +2 Str, +2 Con
Size: Medium
Speed: 6 squares
Vision: Normal
Languages: Common
Skill Bonuses: +2 Endurance
Gladiator Training: You gain proficiency with a simple, military, or superior weapon of your choice.
Great Fortitude: +1 Fortitude defense
Stubborn as a Mul (Mul Racial Power)
You just don’t know when to quit.
Encounter • Immeadiate Reaction
Effect: When you suffer an effect that allows for a save, you can make an immeadiate saving throw against that effect.
Notes: The "standard" races all get +2 to two different "defense" abilities, so getting +2 Str/Con is slightly weaker, although obviously pretty good for a warrior-type.

In any case, a bonus to Dex or any mental stat does not really make sense for the race.
The primary traits of muls are strength and toughness, and in particular that they keep going and going and going... In 2e terms this was related mainly to manual labor, forced marches, that kind of thing. For 4e it would need to be turned into a benefit useful in combat. So, they have better HP, surges, etc (higher Con), a great Fort defense (higher Con + race bonus), and have a better chance to throw off negative conditions with the racial power (stolen from hobgoblins). They're tough, but distinct from dwarves who are tough in a slightly different fashion (higher Con, healing surge as minor action, not easy to move, mostly immune to poison).
The weapon training gives them a little more flavor as the preferred race for gladiators (most muls are born into slavery to fight in the arena), although that might need to be tweaked if superior weapons are significantly better than military weapons.