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The main purpose of Div-1+Div-2s, As I understand, Is to allow everyone have a fair try at winning prizes in a contest with prizes.
I have seen some exceptions to this where the round (1+2) is held without any prizes. But I am not sure about the reason of their existence.

That doesn't make any sense, here's why:

This is a rating number line from $$$0$$$ to $$$4000$$$ (this range covers everyone who's not a troll):

This is the same rating number line with the rated range for each division:

You can see how Div.1 + Div.2 makes sense, a contest rated for all. This is also useful for rounds with prizes so that everyone has a fair chance at getting one.

As you can see, the rated range of Div.3 and Div.4 are already almost the same. Also, there is quite a lot of variation in the difficulty of different Div.3 and Div.4 rounds. In the end, a Div.3 + Div.4 round would just be like any other Div.3 or Div.4 with maybe a few more problems, but what would be the point of that?

is it the same to regular Div3s?

After the div3+div4 round, let's make a div1*div2+div3-div4 round!

What about making div 5 for ratings under 1200? and div 6 for ratings under 1000? and div 5+div6 rounds