Wait, when did WotC produce an E4 stamped Base Set Pikachu? I indeed thought it was custom when I saw you post it… 
Do you have any more information about it? Where you got it from? How it was released? Are there any more known versions of this error (if it is an error)? Are you sure it’s not a custom-made stamp? Etc. First time hearing about this, so as a Pikachu collector and Base Set Pikachu enthousiast, I’d love to know more. Considering the E3 stamp is for the Electronic Entertainment Expo (E3), I’m a bit confused to see an E4 stamp if it is indeed legit.
Greetz,
Quuador