4 BTC on average a day is a bit much, that accounts for 1460 bitcoins a year. In 4 years from now, we would be getting roughly 900 BTC per day and then 4 years after that 450 bitcoins. A loss of 4 Bitcoins per day, may amount to a lot of bitcoins over a 8 year period. We do not know exactly how much is lost and some coins are even burned on purpose. This is like burning money and throwing gold bars in a volcano.
Technically, over time and if bitcoins increase in value, I would assume the average user would
have an even lower amount of btc within a single address.
For example, when a user now buys 1btc for 600USD, that is a lot for them and is probably within
one address. If that user loses control of that address, that user has lost 1btc.
But over time (many years from now) as the "block reward" decreases, the average user will, in theory,
only be able to buy a faction of that 1btc with 600USD, and will likely be in a single address.
If that future user loses control of this address, that user has lost fractions of 1btc.
So, in theory and IMO, over time, less bitcoins will be lost since value will increase and the average users
will only own factions of bitcoins. Now if you want to say how much "fiat" will be lost from those losses,
well that is a different question and could be very massive.
Most bitcoin "lost" are from the early days of mining and should decrease over time, IMO.