hi
i would like to suggest to create single dialer interface and bind all your physical bri interface in that using dialer pool member command.
Also chek out for the dialer load threshold limit also in your dialer interface so that even minimum load can trigger the isdn lines out..
we have sucessfully tried running/bundling 3 physical interfaces and achieved 384K.hope there shuldnt be any probs with 4 links..
do check this link which deals with 2 physical interface which can be referred to bundle all you 4 interfaces.
http://cisco.com/en/US/tech/tk801/tk133/technologies_configuration_example09186a0080093c2e.shtml
for more info do refer this link too..
http://cisco.com/en/US/tech/tk801/tk133/tech_configuration_examples_list.html
regds