Well, the attrition rate is fairly significant for a game like CoH2 that has microtransactions, because having more people playing your game gives you a larger audience to sell your DLC to.
Setting aside the fact that 4 million in raw sales is a fairly impressive figure, if we compare the attrition rate of CoH2 to that of CSGO using some public data, the difference is striking.
Since April 2014, the beginning of this sales data interval,
CoH2 has increased its average daily players count from 3,100 to 5,472. With an estimated 4 million in sales, we can say that for every 1,686 sales of CoH2, it saw an increase of 1 player in its average daily players count.
Compare that to
CSGO, which sold 6.4 million units and saw an average daily players increase from 78,881 in April to 239,935. That amounts to 40 sales per 1 average daily player increase.
1,686 sales per average player increase vs. 40 players per average player increase is a massive difference - if CoH2 had that rate, it would have 103,100 average daily players right now and be the third-most-popular game on Steam - and it shows that CSGO is much better at retaining its sales as active players than CoH2 is. The reason for that is no doubt complex and not easily summarized, but it's still an interesting fact to consider.