Why not? In DTTG3 this was easily possible. One store with the “Download Documents Always” setting and one with the “Download Documents On Demand” setting.
I reported this two months ago, and it was confirmed as a bug. So I‘m hoping it will be fixed soon.
When I first installed DTTG 4 on my iPad - where DTTG 3 was still installed - it migrated my three Dropbox locations to DTTG 4 just fine. But like you, I can‘t add another one now.
I’ve got the same issue on my iPad when I tried to set the different dropbox sync location last week, while on my iPhone I had no problem to add them when I switch to DTT4 end of January. But it seem the option to add a new dropbox location has disappeared now…