Which is better for the beach?

The S4. Its large beach-ready wheels and higher stated capacity handle sand and cargo better than the S2's standard wheels.

Which is easier to lift into a car?

The S2. The S4 weighs 15 kg, so the S2 is the lighter choice for frequent loading and unloading.

Does the S4 have a brake?

No brake is listed for the S4. The S2 lists a brake among its features.

Can I push both wagons?

Only the S4 lists a push handle. The S2 has an adjustable handle for pulling.

Is the S4 worth stepping up to?

Yes if you carry kids or heavy gear over sand and rough ground. The S2 is enough for light errands and tight storage.

Wonderfold S2 Push & Pull Wagon vs WonderFold S4 Push & Pull Wagon

The S4 is the better wagon for hauling kids and gear to the beach or park. It has a push handle plus a telescopic pull handle, large beach-ready wheels, a stated 81 kg capacity and removable fabric for cleaning. The S2 is the smaller, lighter option with a simpler single-handle setup and a brake, and it suits quick errands and tight storage. Owners of both lines praise the folding mechanism; the S4's extra bulk and 15 kg weight make it harder to lift into a trunk.

Choose the S2 if you want a compact, lighter wagon for short trips and easy storage.
Choose the S4 if you need a push-and-pull wagon with big wheels and room for kids and cargo.

Prices

Wonderfold S2 Push & Pull Wagon: $149.00 at Campingworld (checked today)
WonderFold S4 Push & Pull Wagon: $149.00 at Bass Pro Shops (checked today)
Difference: both cost the same today