You could say:
awk '{NR%2 || $5="\\n"$5 }1' filename
Note that you'll need to escape the \
in order to get a literal \
.
For your input, it'd produce:
2, 0, 0, 0, 2
4, 0, 0, 0, \n4
6, 0, 0, 0, 6
8, 0, 0, 0, \n8
Alternatively, (as pointed out by @WilliamPursell), you could say:
awk '!(NR%2) {$5="\\n"$5 }1' filename