One cell?
You could use a formula:
=--text(a1,"0000\/00\/00")
and give that cell a nice date format.
If you had a column of these cells to convert...
Select the column
Data|text to columns (xl2003 menus)
fixed width (remove any lines that excel guessed)
choose date ymd as the formatt
and finish up
If it were 4 hours (60 minutes times 4), you could put 1440 in an empty cell
(1440 = number of minutes in a day (24*60))
copy that cell
select the range to fix
Edit|paste special|choose divide and values
And then format the cell with a nice time format like: hh:mm:ss