schema doubt

Hi

I have a scenario where I have 4 child elements(a,b,c,d) within a parent
element X. a,b,cd can appear in any order within X and any number of
occurences.

I want to know how to write a schema file meeting the above requirement.

Note : I tried "all" data structure but then it allows 0 or 1 occurence of
a,b,c or d. This does not meet my requirement. I need to have flexibility of 0 or more occurences of a,b,c and d.

Any suggestions will be of great help. But please make it fast. Very urgent.

Regards
Deepa