Is the additional payment only for the hours worked between midnight and 4am or for the whole shift?

If the worker only crosses into this 4 hours period for say half an hour, at either end, does the whole shift accrue the extra payment?

Also, are you looking at a one off extra payment if the shift crosses into this 4 hour period or an additional hourly amount?

Sorry for all the additional questions but the original post was a little vague on details.