2006: 5.3 million sales:
https://web.archive.org/web/20090606133308/http://www.konami.jp/gs/game/silenthill/

>SH
>SH2
>Sh3
>Sh4

5.3-2 = 3.3 MILLION sales
=1.1 million sales for 2,3,4.

2009: 6.9 million
https://web.archive.org/web/20110506044206/http://www.konami.jp/gs/game/silenthill/

That includes Oriigns, Shattered Memories, and spin-offs.

Those are DREADFUL numbers.