The P4-compat module allows you to process existing P4 conformant data with a P5-conformant schema. It does this by:

• re-instating features which exist in the P4 core but not the P5 core
• removing features which are new in the P5 core

For this to be possible

• P5 elements which are synonymous with P4 elements may extend their definitions but not restrict them
• in P4 compatibility mode, P5-specific features are disallowed
• P4 and P5 instances must be processed separately

All this is feasible using current ODD system (with some usage rules, e.g. for xml:id)