It’s “one or more letters” followed by one digit, repeated four times. I found what I was looking for:

([\u]+?\d)([\u]+?\d)([\u]+?\d)([\u]+?\d) replaced by $1$2$3$4=$1+$2+$3+$4

I see how powerful GREP can be and will do my homework on getting the syntax right. Thanks a lot for the quick reply.