4 wire high leg has one of the transformer secondaries center tapped, with the center tap grounded. 3 wire delta does not have the center tapped secondary.
If you have 240V service, then 4 wire delta gives you 240V phase to phase, 120V from the A and C phase terminals to the grounded center tap, and 208V from phase B (the high leg) to the grounded center tap.
With 240V service, 3 wire delta gives you 240V from phase to phase, with no other voltages available.
-Jon
