Try this awk
:
awk -F'"' 'NR%2{p1=$4;next} {print p1 "https://stackoverflow.com/" $4}'
Test:
$ awk -F'"' 'NR%2{p1=$4;next} {print p1 "https://stackoverflow.com/" $4}' file
abc/123abc
bac/bac123
cde/cd123
b4u/b4u234
2
solved grep values and re-arranging the file