Given it's half-life and a arbitary time-line and your desired dose I would take:
06:00 15mg (first day take 20mg)
10:00 10mg
02:00 10mg
06:00 10mg
10:00 10mg
I know this is 55mg, but would be ideal as far as the 4 hour half life, a breakdown of the time and blood level (not really that accurate, but should be close) would be:
6:00 20 (take 15mg as you still have some present)
7:00 17
8:00 14
9:00 12
10:00 20 (take 10mg)
11:00 17
12:00 14
1:00 12
2:00 20 (take 10mg)
3:00 17
4:00 14
5:00 12
6:00 20 (take 10mg)
7:00 17
8:00 14
9:00 12
10:00 20 (take 10mg)
11:00 17
12:00 14
1:00 12
2:00 10
3:00 8
4:00 7
5:00 6
6:00 5
Have you tried it before? 25mg is usually enough for most. I personally would run 30mg - 10mg upon waking, and 5mg every 4 hours.
Other option would be 20mg upon waking, 10mg 8 hours later, 10mg before bed. This will keep your levels rather even and higher first thing in the morning for your workout. Weekend you might take the 20mg before "bed" or at whatever time you want that little extra.
I think that if you are going to do it, run it every 4 or every 8 to keep blood levels relatively high/stable.