A supposed 4DS, or whatever you would call it, is no longer viable at this point in time.
They will keep the 3DS around through 2019 to milk it dry. And, then they will release the Switch Mini (possibly Fall 2019) for $200 to take its place.
They will not release a separate handheld with games that do not run on the 2DS/3DS or the Switch anytime before 2023.
Right now, 2DS is $80, N3DS XL is $200, Switch is $300, There is not enough room there, price wise, for a seperate non-Switch based "4DS". Especially as the switch gets cheaper each year.
Nintendo have cornered themselves by having a product like the Nintendo Switch that covers so many advantages:
-TV out
-Portable Splitscreen (which is also organic advertising)
-Many Controller Options
-Existing game library will be large by then
-etc.
As the price drops on the Switch, they will only be able to launch a new handheld that exceeds all of the advantages of buying a Nintendo Switch or buying a 3DS. (an example of such a possible handheld is, as many have mention, a Switch mini)
Anyway, with all that Nintendo has communicated about wanting software compatibility across many devices, the switch serves that role.
Right now, the Switch is hybrid hardware, another Switch SKU can be dedicated home console hardware, while another Switch SKU can be dedicated handheld hardware, etc. All sharing the same game library, same OS, but with different form factors.
Having to only make games for one system, with many form factors, eliminates redundant development costs. This also allows Nintendo to be more agile in having a regular release schedule of games, which they irrefutably struggled with both the 3DS and Wii U years on end.
It is likely that the Switch will outsell both lifetime hardware and software totals of the 3DS and Wii U combined. All while reducing manufacturing costs, development costs, and marketing costs as they only need to target 1 platform. Nintendo is diversifing in other ways with Android/iOS games, cards, amiibo, other liscenced merchandise, theme park, etc., etc. so their eggs are not actually in only 1 basket.
Anyway, The 3DS can only live for as long as people keep buying it. The 3DS is reaching lifetime saturation even with all of its revisions keeping it just above water.
Getting past 80m 3DSes would take several years more if Nintendo was that persistant, as YoY sales will drop as more and more now, people are replacing their 3DSes with Nintendo Switches.
