That's quite high for a consumer product. The standard for most consumer receivers is about max of +2dBV ( ~1.25vRMS) and nominal of -10dBV ( ~315mvRMS). This is the level that the -10dBV switch position assumes in the BFD.
The +4dBu switch position in the BFD assumes a max of +16dBu ( ~4.9vRMS) with a nominal level of +4dBu ( ~1.25vRMS).
So, you can see that your BFD would be better suited for the +4dBu switch position.
If no other information is given, it's hard to know at what trim position of the sub control on the receiver that the specification of 4volts is given in your manual. If we assume it's the output max at mid position (zero) of the trim, then if you have the trim at -10max cut, then you should not be able to clip the BFD using the +4dBu switch position.