Firstly, I'd like to say that Div. 4 contests are a great addition for people who have absolutely no idea about competitive programming and the ability of being able to solve more than just one problem will definitely make new people more confident when it comes to problem solving.
However, this change will definitely bring lots of new contestants to Codeforces, and some measures will have to be taken in order to deal with the new change.
As Errichto suggested, a rating lower bound would be beneficial because it partly solves the rating inflation and also because it will generate less queue.
Starting from this point of view, I'd like to suggest further improvements which can be made in order to make the platform even better:
1) Since we have parallel div1/div2 rounds, maybe we can also have parallel div2/div3 and maybe div3/div4 parallel rounds as well. Obviously, some lower bounds will have to be implemented, but this is not much of an issue, because the current lower bounds for colors should make the splits easy to implement.
2) Maybe let other people become problemsetters for div3 as well, there are plenty of people who want to become Codeforces problemsetters but maybe their tasks are not considered good enough for a div2 round because of one reason or another. Since div3 tasks are meant to be more standard, the problemsetting criteria can be less harsh.
3) Given the recent inflation of ratings, which is also a problem and if none of these suggestions will be implemented, this is going to become a major problem, since one can become purple with div3 rounds(look at andryusha_na_knopke for proof), maybe also change the color thresholds for each color.
What do you think about these possible new changes, would they be useful to make the Codeforces experience even better in this era of dramatic growth? I'd like to see your opinions in comments.