The thing about the PS3 is that it had a backloaded sales curve unlike the PS4, so in 2013 (which is the comparable year to the PS4's 2020) it still managed to sell 8.25m units globally despite lower peak sales than the PS4 pulled off. In 2012 the PS3 sold 12.15m.
The PS4 sold 14.2m in 2019 and if the currently estimated 40% decline holds throughout the year, then that's ~8.5m in 2020. So heading into the post-replacement phase, the PS4 is selling similarly to the PS3. Since some time the big question has been if and when Sony will give the PS4 another price cut. If the PS4 gets cheaper, then it can do a bit better than the PS3 in terms of post-replacement sales (say, 10-12m instead of ~8m). But if it doesn't get a price reduction, then it's looking likely that we'll only see PS3 levels.
I guess I still expect it to get a price cut at some point as unlike the PS3 it doesn't use expensive proprietary parts and they surely will have the leeway to cut it to $250 or $200 before they discontinue it.
Bet with Liquidlaser: I say PS5 and Xbox Series will sell more than 56 million combined by the end of 2023.