Having a TTFFTTFF... pattern is quite easy:

  DB<11> $i = 0; say for grep { $i++ % 4 < 2 } 0..12
0
1
4
5
8
9
12
[download]

And starting at the fourth value in the pattern just means your start at the fourth value of $i:

  DB<12> $i = 3; say for grep { $i++ % 4 < 2 } 0..12
1
2
5
6
9
10
[download]