I've been 4e since the get go, and with two gaming groups- one via Maptools (5 or 6 players) and the other real people (always 5 players).
I'll mostly concur with Wedgeski and Tamlyn as regards some one turning up late, or all of the players (on my face to face game arriving 5-10 minutes late)- but that's just life.
Here's my breakdown for 4 Hours-
Maptools Heroic- RP breaks down into 30 min chunks, sometimes it takes x2 chunks to get things done. Fights are generally an hour on average. However of late we've been losing at least 30 min to technical issues every session- average two fights and a chunk or two of RP, depending on late arrivals and comms breakdowns.
Face to Face Heroic- RP gets done a little quicker, say 20 min chunks, although as above it sometimes takes 2 chunks to get stuff done. Fights are less time time, 45 minutes certainly for low level Heroic- some much quicker. So average out three fights and some roleplay, although we often play longer sessions around the table and get lots done.
Maptools Paragon- fights last at least an hour, more likely 90 mintues- so one or nearly two a session, what with lateness, comms probs and RP.
Face to Face Paragon- fights last longer again, about 75 minutes I'd say- so two or three fights and some RP.
Full breakdown of actual sessions are in the stat Story Hours in my sig for KOTS, Thunderspire & Sellswords.
Cheers Goonalan