"The fourth wonder is: the fountains discovered in the same [place] of salt, from which fountains salt is cooked: from that place diverse plain-foods are salted and they are not near the sea, but from the earth they emerge."1,2
This Wonder is a geological one. Salt springs3 are rare relative to freshwater springs away from the coast, but are still more common than you might imagine. For a society, like our compiler's, that relied on springs in part for its water, they would certainly be more obvious than today - firstly as they would be undrinkable, and secondly because they would have provided salt for preserving meat. The fact that salt water was welling up so far from the sea may well have seemed wonderous given the ignorance at the time about sub-surface geology.
The "same" in this Wonder's description refers back to The Third Wonder - the Hot Pool in the territory of the Hwicce. Because of this the most likely candidate for this Wonder is the three old salt springs at Droitwich4,5, where there has been an uninterupted tradition of salt spring use from the Iron Age, through the Romans and Saxons, to today.
You can find out more about the history of Droitwich Salt Springs here. You might also like to read more about the Myth and Science of this Wonder, or see some Visit Details.