Providing the specialist has the diagnostic tool which will tell them which corner is affected, there's no need to go main dealer. However if they potentially have to change all four due to not knowing which one is at fault, it may end up more expensive than the main dealer.
A faulty ABS sensor will mean that the ABS and tyre pressure monitoring systems are not working correctly and the ASC/DSC
(if fited) as they all run off the same sensor. It's a safety issue, and could potentially cause insurance issues if it ends up causing an accident
I'd get it sorted ASAP.