When using code like this:

var updateDefinition = Builders<MyDocument>.Update
  Set(...)
  Set(...)
  Set(...)

A tree of CombinedUpdateDefinition<MyDocument> will be created, Each one of them containing a List<UpdateDefinition<TDocument>> (which will hold a 4 element TDocument array).

Combining with a CombinedUpdateDefinition<MyDocument> should be special cased to add to the existing CombinedUpdateDefinition<MyDocument> or, at least, create a single new one.