Think of the L5 as an L4 on steroids. Similar flood, but with much more throw (as ASDalton and JonSidney correctly indicate).
Being an E-body, the nifty L4 is more "pocketable."
The L5 is heftier and built for any tough duty you can toss at it... including tactical illumination alongside a heavily-recoiling handgun. In fact, I understand that the L5's big brother (the 3 X 123 L6) is now a favorite of many combat pistol-craft instructors for night shooting (exercises... and the real thing). Still, the L5 is plenty adequate for nocturnal confrontations of the worst kind -- and it sure beats the big old Mag-charger I used when I took my first night shooting course 20 yrs. ago.
The L4 probably has slightly better (flatter) "regulation" than the L5 in that most tests show it running flat for about 75-80 minutes, whereas the the L5's heavier drain on CR123 lithium batts will cause it to begin to fade at about the 60 min. mark. However, the L5 will then give you another 30 min. of useful (though diminishing) light. I've found this diminishing "tail" light from my L5 to offer still decent/useful flood... but rapidly "shortening" throw.
Another L5 advantage is its ability to take a rechargeable Pila 168S battery.
In summary, the L5 trades about 30% of the L4's flat runtime for a 75% + increase in throw for about an hour.
Both are great lights. However, the L5 is one of the few LEDs that have enough punch (throw) to work well OUTSIDE (in urbia/suburbia) where competing ambient light clutter cuts most LEDs' effectiveness.
My L5 has become my main Night-Walk light. I accepted that it would chew through an additional $15-20 worth of 123 batts per annum compared to some longer-running lights, but I figure the enhanced "situational awareness" and intimidation/safety benefits it provides are worth it. I occasionally turn to my Pelican 3W LED (longer throw, but weaker flood) or a SF C3 incan (almost TOO bright) when humidity/rain/snow/fog conditions require even more punch, but the L5 cuts-the-mustard best 90% of the time.
"Sean's Flashlight" website (a fellow CPFer) has some great beamshot photos comparing the L4 and L5. They speak a thousand words.