Break it up into smaller sections and composite them all together. If you render for say 8 hours overnight and view it then you can make sure everything is on track and in place. Otherwise if you wait till the 4 days are up and notice an error in the first say 400 frames you'd be pretty upset I'm sure.
Plus if you have checkpoints it allows for modification of further possible errors so the next render sessions go off without a hitch. And finally what if during the 4 days the power goes out? That would kill all your work up to that point right?

Good luck!