Step 1: Look at the Cute Little Mouse Toes! (background)
Toe clipping is one of many methods of mouse-numbering used over the years in large lab colonies. The IACUC now recommends against toe clipping, instead preferring techniques that are less invasive and/or require lower levels of training. Properly clipped toes are easy to read, and make for unambiguous numbering. Even though you'll probably never need (or want) to clip a mouse's toes, the counting method is still of interest.
When you hold a mouse, they quite obligingly splay their toes for you to observe and count. (Because I don't keep rodents anymore, I found this nice picture on the internet.) They have four fingers and a thumb. This numbering method utilizes 1-2 fingers per paw, leaving the thumb intact for proper mouse motility.
Step 2: Scheme
Thus, the front right paw is the "ones paw", 1-9, (100); the front left paw is the "tens paw", 10-90, (101); the rear right paw is the "hundreds paw", 100-900, (102), and the rear left paw is the "thousands paw", 1000-9000 (103). Mix and match.
The mouse is held as shown, with splayed toes facing the reader. Toes are read left to right in all cases.
The following demonstration only uses the 1's and 10's paws (since I can't do much with my hindpaws) but hopefully you can extrapolate.
Step 3: Count to 9
1-4 are obvious, right? Then you move onto combinations of two, walking along left to right. Follow along on your own fingers if the progression doesn't immediately make sense.
Step 4: Count by 10's
Step 5: Test
Now use this technique to signal your friends across a room.