Try this
=(A4-INT(A4))*24
Make sure column A is HH:MM then put the formula in column B, (This can be hidden later)
You can add the figures in column B normally as they are decimals, lets say..
=sum(B1:B10) this is in B11. If column A11 = B11, you will get a HH:MM total.
I'm sure that there are other ways, but this works.
Mutt