This is by design as far as I'm aware. In order to add a mask for only numeric digits (and only a certain number of them) you could do the following:
.radMaskedEditBox1.MaskType = MaskType.Standard;
This would ensure that the user can only type in 4 numeric digits.
The documentation for the mask editor can be found here
and the subsections detail the types of masks and how to use them.
Hope this helps, but let me know if you need further assistance