$ awk 'NR%4==0 { print $0, "in line#", NR }' infile
a in line# 4
b in line# 8
c in line# 12

so:

awk 'NR%4==0' infile

$ awk 'NR%4==0 { print $0, "in line#", NR }' infile
a in line# 4
b in line# 8
c in line# 12

so:

awk 'NR%4==0' infile

change to awk 'NR%4==1' if you want print the first line then skip every next three lines.