Pierre-Luc,
If all your text/string fields have the same format, P4593, for example. Then you could use the Right VB syntax to extract the 4 right most characters. In this case my formula would be:
NewField = Right ( [StringField], 4)
This would extract the right 4 "characters" --- 4593 --- and put it into the Double field. If the [StringField] is like P45931, then you would update the formula to be:
NewField = Right ( [StringField], 5)
Then 45931 is calculated into the NewField field.
Hope this helps!
Robert LeClair
Esri-Denver