They are 6 points away from the club in the top 4 currently occupied by Leipzig. That's a pretty tiring distance to chase, but considering the number of matches remaining in the Bundesliha there are still a lot of points that are still very much in the chase. I think to be in the top 4 is still very doable, but I doubt if chasing the top of the table, because their point gap is now far away, and with the appearance of the club that occupies the top position, it looks like an impossible thing to do.
Indeed in football nothing is impossible, as long as there is still a chance as long as they still have a chance, but we also have to be realistic with what is happening at this time. Moreover, the ones they are chasing are also among the clubs that dominated the league for more than a decade.
Dortmund is in the 5th position right now. The past few weeks have been pretty bad for them. In the last 5 matches, they have only 1 win. They have 3 draws and one loss other than that. That is definitely not good enough from them. Now if I were asked if they will be able to finish in the top four or not, I would probably say that it is possible, and maybe even probable. But the way they are performing right now, it doesn't seem like they will be able to have consistent performance in recent future. So I wouldn't be so confident about them as well.
Yes it is true, Dortmund's performance lately is quite a concern, usually we see that they are always competing at the top of the standings with Bayern Munich in the race for the title, but specifically this season there really is a change that might be said to be quite significant from this team. As you said and it's a fact that in the last five matches they were only able to win one win three draws and one loss, honestly on the other hand what I think is unexpected is when they played in their last match against Mainz, although sharing points but honestly it's really a bad result for a team like Dortmund who is clearly favored when dealing with a team that is almost in the relegation zone.
On the other hand for the problem of what position they will finish I think I can't be sure yet, as you said the fourth position is something they might be able to get, although it is quite doubtful but I think it looks like they can finish in fourth position because after all there are still quite a lot of opportunities that they can take advantage of. Next up they play Darmstadt, overall they should be able to get full points against a team at the bottom of the table, but there's no denying that a bad result like the one against Mainz in the previous game is still possible.