Any reason you are using a dealer?????
Ima gonna guess you though "I'll take it to the dealer cause they are experts...and sure they may be expensive but at least it will be done perfect. $3900 is a lot, but hey- they never make mistakes. And if they do, they stand behind it"
I hope I am wrong, but IMO- get ready to hear more lies...
Oh, if you want help you need codes and the results of test plans. And a very clear idea of what they did (ie testplans from prior to repair, documented work done with part list, etc.) Please dont approve any work without checking back in. Remember, BMW service advisors are absolute professionals at separating you from your money- first and foremost. You will be given information in a way that serves THEIR needs, not yours.
Edit: If you have the work order, black out your stuff- so we cannot see VIN, names, numbers or dealer stuff- and post it. Part numbers, and work done.