E2 = dashed magenta border that actually surrounds the airport whose approaches are being protected = "6002. Class E Airspace Areas Designated as Surface Areas. The Class E airspace areas listed below are designated as a surface area for an airport."

E4 = dashed magenta border that does not actually surround the airport whose approaches are being protected but rather adjoins some other airspace that surrounds that airport = "6004. Class E Airspace Areas Designated as an Extension to a Class D or Class E Surface Area. The Class E airspace areas listed below consist of airspace extending upward from the surface designated as an extension to a Class D or Class E surface area.

E2 = dashed magenta border that actually surrounds the airport whose approaches are being protected = "6002. Class E Airspace Areas Designated as Surface Areas. The Class E airspace areas listed below are designated as a surface area for an airport."

E4 = dashed magenta border that does not actually surround the airport whose approaches are being protected but rather adjoins some other airspace that surrounds that airport = "6004. Class E Airspace Areas Designated as an Extension to a Class D or Class E Surface Area. The Class E airspace areas listed below consist of airspace extending upward from the surface designated as an extension to a Class D or Class E surface area.

Source of quotes -- the FAA's "Airspace Designations and Reporting Points" document, current edition FAA Joint Order 7400.11E, effective September 14, 2020. All the entries in the relevant sections start with "E2", or "E4", as appropriate.