BPF filtering on more than 3 ports?

Question

Hi there,
I have a piece of software which uses BPF filtering on the command line. The packets that I am interested in always have either a source or destination UDP port from the following list:
o 1645
o 1646
o 1812
o 1813

Unfortunately, my BPF syntax can only filter a maximum of three ports:

udp port (1645 or 1646 or 1812)  

When I try to filter more than this, for example

udp port (1645 or 1646 or 1812 or 1813)    

the filtering begins behaving abnormally.

What would the correct BPF syntax be to filter out all traffic which is NOT coming from these ports?

Thanks,
Rik