While 4 million copies across 3 games might sound a lot you have to bare in mind that the revenue from each copy is nowhere near the revenue from console games.
The majority of those STALKER copies were sold at only $15 a piece, and even $10 in Eastern Europe where the majority of sales for STALKER are.
Each copy sold brings $10 if very lucky (a console game gives $25), which means only $40 million in total revenue over 6-7 years of development. And certainly publisher THQ must have snatched some of that. Fortunately the developer is based in Ukraina because that revenue simply wouldn't have been enough for a Western developer to pay its workforce.
Nowadays apart from Blizzard the only non-MMO big budget PC exclusives are made in Eastern Europe due to the lower dev costs. Such is the state of the PC gaming market.
