1 in 4 likely means you are on call 24 hours a day for 1 week out of each 4 weeks. The support is then shared between 4 people. That's a fairly typical arrangement in largish companies.

You had to make contact within 30 mins when paged/called and be working on the problem within 2 hours.

1 in 4 is being on call 1 week in 4 though some DBA's where on 4 in 4 and got called about twice a year very lucrative.