I don't liked RE4 to the point to never buy/try RE5 and RE6.
Loved RE7 and plan to buy RE8 too (yeap it is weird but I'm waiting PSVR2 version).
The point is the change in focus to action instead survival started in RE4... it bored me to even sleep playing the first 30 minutos of the game (that people says it is good)... the change was not good.
And while people take RE4 as the "masterpiece" it sales showed the opposite... it is one of the worst seller RE that only get high combined sales because had like 10 RE-REleases... when the best seller version of the games is a PS4/XB1 re-re-re-released and only sold around 2.5m you know there is something wrong.
In that regards I think RE5 is somehow a better game because it has a clear focus based in what RE4 changed.
But that is me.
RE for me is still RE1-3 plus CV and now RE7.
I don't know if I agree with that. Action started becoming more normal in the series with RE3. What RE4 did do is focus less on being a puzzle game with limited ammo and not being able to see you enemies in the distance and focus more clearing enemies and getting from point a to b.
I would consider RE4 to be about survival but I think it's just easier to master due to the nature of not obscuring things in the distance.
But the thing you said about its sales is that RE4 didn't have similar launches to RE5 or RE6 in anyway. It released on Gamecube two months after the PS2 port was announced. Look at that old Neogaf thread from November 1st, 2004 and see how people reacted lol.
and The PS2 port released close to the end of the PS2's Life (a month before the release of the Xbox 360) as well in a generation where digital sales are not a thing.
RE5 and RE6 seemily still sell 100k on PS3/360 every quarter but all of that is most likely digital sales and not physical copies which the early version of RE4 don't have to luxury of having. (The Wii Edition does seem to include Wii U eShop sales since the platinum listing has "DL" next to it but it wasn't enough to push the Wii Edition listing up 100k since you know the Wii U was well... the Wii U!)
The first version of RE4 that got the luxury of Digital sales to boost them was the PS3/360 versions and that released when Digital Sales were no where as big as they are today and RE4 on PS3/360 only got a physical release in Japan and no where else! (Not even the 2007 PC version was released digitally!)
Steam and the PS4/XB1 remaster are really the only ones comparable to how RE5 and RE6 released since the PlayStation and Xbox Port launched Day and Date with each other (11 and a half years after the original release.) and the Steam just about 2 and a half years before. RE5 and RE6's PC ports where just months after the original release as well!
Also don't forget RE5 has a gold edition, an Nvidia Shield port, 2 PC ports like RE4 (Microsoft Windows, and Steam.)
Also RE4's 11.8 Million sales don't include the PC 2007 port, Mobile 2008, Zeebo, Ios, Android versions
The Steam versions of RE5 and RE6 have not hit 1 million while the Steam port of RE4 hit 2 million last quarter and The PS4/XB1 port of RE4 has sold better than the PS4/XB1 port of RE5 and RE6 despite both of those release before RE4 on those systems.
It will be interesting to se which game hits 1 million on Switch first but this might take longer than excepted because I wouldn't be surprise if the Digital versions of RE4 is going to be counted separate from the Triple Pack physical release the Switch got since digitally RE4 released on Switch on May 21st, 2019 and released again on Switch in the Triple pack later in the year when RE5 and RE6 made their debuts with the triple pack!