Assuming you are near a bank (Fishing Guild or Catherby)
Lobby's = 90 xp,
Say you fish a full inventory of 27 every 5 Min's = 2430 xp
I hour = 2430 X 12 = 29160
3hrs= 3 x 29160 = 87,480
4hrs= 4 x 29160 = 116,640
Therefore between 87- 117k of fishing xp
In one hour at above rate you would catch = 324 lobby's
In three hours you would catch 972 lobbies
In four hours you would catch 1296 lobbies
Selling them uncooked as you state at GE price of 310 ea currently
In 3 hrs you would get approx 301,320 cask (approx 300k+)
in 4 hrs you would get approx 401,730 cash (approx 400k+)
THIS IS ASSUMING YOU GET A FULL INVENTORY EVERY 5 MINS
**IF YOU FISH A FULL INVENTORY EVRY 10 MINS THEN YOU NEED TO HALF THIS ESTIMATION**
A good site to help you with other Runescape matters is,
http://www.go-runescape.com