I would try to negotiate a lower labor rate with them. 4 hours is WAY too much time for this job. There are two types of sensors and both need programming once installed. It might help if you tell them you know how much work is actually involved in installing the sensors but some dealers just don't budge on the labor rate booked for each job.
It sounds like they are replacing one of the two door tilt sensors ($70 at Tischer) since it would take a bit of work to remove the inner door panel to replace this sensor. Maybe 30 minutes to an hour work. There is also one of these same senors located in each C-pillar which would take only a few minutes to replace.
There is a combination siren/tilt sensor behind the rear bumper ($150 at Tischer) which would take less than 20 minutes to replace and since it costs $150 at a discount dealer I don't think that is what they are replacing - unless they are giving you a very good deal on parts pricing to make up for the crazy labor rate.
Here are pics of the two sensors and the part numbers.
[edit] hmm strange, Bimmerfest is now blocking links to the bmwfans parts site, only realOEM works. Too bad since the bmwfans site has more up-to-date ETK diagrams.
Instructions for a door sensor install.
Instructions for C-pillar install.
Instructions for bumper install.
If you are handy you could install these yourself. The only problem is the coding. A BMW specialist indie mechanic would probably charge you less than $100 to install and program one of these modules.