Laurie: I am going to weigh in here, and hope that I can contribute some helpful advice. I have been a DREMEL user since the mid-1960s and I have also been (proudly) a DREMEL product demonstrator since 1996. And I still AM!!)
Okay. I have not yet seen or used the new Model 4000. So I cannot/will not comment upon it.
If the 4000 did not exist, I would (and have) advised any of my students to have bought the 400XPR. 2.0 amp power means lots of rotational power (torque) at low speed which is good for sanding and drilling. At high speed, lots of torque means "feather touch and very precise work"
The 400 XPR will do more work under less stress than it's 1.16 amp cousin the 300 series.
Both have brush-type motors and a good warranty. So you won't be chucking your tool out soon.
Whatever tool you buy, make sure you get a Flex Shaft Attachment.
Hope this helps!
"For almost every problem,
there is a DREMEL solution"
"When all else fails, read the instructions."