How do only start four digit and after decimal keep 3 digits into asp.net

Hi,

Our client is required such type. I have taken one textbox and i have used MaskedEditExtender. But this ajax contro is
not accepteding backspace only allowed take cursor first into textbox on starting then allowed to type. but this is not.
If i press tab then it is going that textbox and my cursor is coming last point and I try to remove that string it not allowed
backspace..

e.g.
1000.001
0.001
0.034

Any other masks or formt, so i can use

thanks..