For this you need the TIMEVALUE() function.
This takes a Date/Time cell and converts it to a value representing the time (expressed in 24 hour increments), so 12PM would be 0.5, 9 AM would be 0.375, etc.
Once you have these values, you can subtract one from the other to find the time difference. Multiplying this by 24 gets you the number of hours.
For example, if your formula is:
=(TIMEVALUE(G4)−TIMEVALUE(F4))×24
you'll get the time difference (in hours).
Note that because you're stripping the date, this ONLY works for same-day values - if someone is working a graveyard shift, say 10PM to 6 AM, this would return a negative number.
if that's a problem you can add an additional IF() function to check for negative values, and adjust accordingly:
> IF((TIMEVALUE(G4)−TIMEVALUE(F4)) <0,24+(TIMEVALUE(G4)−TIMEVALUE(F4))×24,(TIMEVALUE(G4)−TIMEVALUE(F4))×24)
If the time differences ever span more than 24 hours (what kind of boss are you! :-D) you can use DATEDIF() to calculate the number of days difference, and add that into the mix, but it doesn't sound like that's needed here.