ircstat/stat, ircstat/mlstat: print month/year only every 4th month

While still readable in the X11 output, the month/year labels got too
close in the PNG output. We thus only print them every 4th month now,
instead of every 3rd.

ircstat/mlstat

@@ -21,7 +21,7 @@ while read l; do
 	esac
 done <ML
 
-N=3
+N=4
 t=
 >_out
 n=0

ircstat/stat

@@ -11,7 +11,7 @@ case "$1" in
 	exit 1;;
 esac
 
-N=3
+N=4
 t=
 >_out
 n=0