There are two main types of bb bearings, cartridge and adjustable type.
Your Defy 4 frame will have British thread. The other bb parameter necessary to ensure compatibility with the frame is bb shell width. On the other hand, the bb parameters necessary to ensure compatibility with the chainset and chainline are spindle length and spindle type (such as ISO or JIS square tapered, hollowtech I or II, isis, Powerspline etc.).
It appears some Defy 4 have Truvativ chainsets, some Truvativ chainsets are only compatible with specific proprietary Truvativ bb's (e.g. Powerspline), which might mean you have no choice of bb or bearing type except whatever they offer.
Unless you have the spec of the existing bb, you will need to know the frame and chainset/chainline requirements to source the right bb.
