When you say is "four full days enough for DL" do you mean to see and do everything in just DL or do you mean in both DL and DCA? I ask because I think you can probably see most everything in DL in four days, but you can't see most everything in both DL and DCA in four days.
There is a chance that the first of the Mickey's Halloween Parties may start on September 22 (Friday). They haven't announced the dates for the parties yet (historically, that's happened in mid-summer/mid-July), so that's something to keep in mind. DL's MHP has the same basic elements as MK's
MNSSHP, but the two parties are pretty different. We don't know much about this year's MHP, but if it's similar to the 2016 parties, then I wrote up a comparison of the two parties in
this thread.
By the time you go in September, DLR should be celebrating HalloweenTime, so be sure to check out the attractions with holiday overlays (e.g., Haunted Mansion Holiday) and any decorations and activities they have. Weekday operating hours and entertainment may be reduced in September and there is a chance that you won't see any fireworks (often only on weekends) or F! (possibly, although the schedule is not yet known). If there is a MHP on Friday, then there won't be fireworks or F!; if there isn't a MHP, then I would consider adding Friday to your DLR days so you can check out the nighttime entertainment.
Another show to look into is Mickey and the Magical Map. It is not presented daily during the non-peak season. I think it's usually dark in the middle of the week, but check the schedule to be sure. The operating calendar is usually posted about 45-days in advance, with entertainment (and refurbishment) being added after that time. Dining reservations open up 60-days in advance, but it's not as critical to be online right when a new day opens. If you find something is unavailable at the 60-day mark, it probably means they haven't yet loaded it into the system (this happens not uncommonly, especially at the start of a new month or a new "season").
If you haven't been in 10-years, then DCA has undergone a fairly major refurbishment since you've last been there and it would be worth considering a visit to that park. Carsland and the redesign of Buena Vista Street happened within the last 10 years (Carsland is one of my favorite lands within DLR; I think they did a really good job of capturing the look of Radiator Springs). They've also just opened GotG Mission: Breakout, and last summer debuted a new Frozen show (live at the Hyperion). They've also redone the area around Soarin' (cosmetic changes mostly, but fits in nicely with the attractions). DCA also has WoC and the Pixar Play Parade, which are unique to DCA.