The 4" pipe in the store is almost certainly Schedule 40 PVC. That is used for Drain/Waste/Vent as well as pressure pipe.
Plumbing fittings are marked DWV (Drain/Waste/Vent) to indicate that they are not pressure fittings.
The on-line place may be selling Schedule 40 pressure fittings. They are not acceptable for DWV because they don't have the smooth internal flow passages required for DWV. They are more expensive because they contain more material and they are rare.