The Webley Junior first appeared in 1930 and changed in 1931 to Blued metal grips, Value is anything from £50 to £150 depending on the state it is in.
The number 4 was added to the Birmingham address on the first models to appear after WW2 on pistols with the numbers J21551 to J25065.
If you can find any more markings I might be able to tie it down to a year for you, email me if you want more info.