Isohalines are lines that join points of equal salinity. Salinity refers to the concentration of dissolved salts in water. It is measured in parts per thousand (ppt) or practical salinity units (PSU).

Option 1: Temperature is measured using isotherms, which are lines that join points of equal temperature. Therefore, isohalines do not represent equal temperature.

Option 2: Pressure is measured using isobars, which are lines joining points of equal atmospheric pressure. Since isobars are not isohalines, isohalines do not represent equal pressure.

Option 3: Rainfall is the amount of precipitation, such as rain or snow, that falls in a specific area. Isohyets are the lines that join points of equal rainfall. Isohalines do not represent equal rainfall.

Option 4: Salinity refers to the concentration of dissolved salts in water. Isohalines are lines joining points of equal salinity, so option 4 is correct.

In summary, isohalines represent lines joining points of equal salinity, not temperature, pressure, or rainfall.