I'm trying to write a regular expression using C#/.Net that matches 1-4 alphanumerics followed by spaces, followed by 10 digits. The catch is the number of spaces plus the number of alphanumerics must equal 4, and the spaces must follow the alphanumerics, not be interspersed. 

I'm at a total loss as to how to do this. I can do `^[A-Za-z\d\s]{1,4}[\d]{10}$`, but that lets the spaces fall anywhere in the first four characters. Or I could do `^[A-Za-z\d]{1,4}[\s]{0,3}[\d]{10}$` to keep the spaces together, but that would allow more than a total of four characters before the 10 digit number.

Valid:
`A12B1234567890
AB1 1234567890
AB 1234567890`

Invalid:
`AB1 1234567890 (more than 4 characters before the numbers)
A1B1234567890 (less than 4 characters before the numbers)
A1 B1234567890 (space amidst the first 4 characters instead of at the end)`