Electrical conductivity of materials can be measured by either 2 probe or 4 probe. The 4 probe technique is widely used for materials having metallic or semiconducting nature. For dielectric (Insulating) materials, two-probe measurements are preferred.

Ultimately, selection of technique depends upon the nature of the material.

You can also utilize Impedance Spectroscopy to measure the conductivity of sheets.

Thank you all for your replies, I greatly appreciate the help and advice :) 

Thank you very much Dr. Tsanov, I greatly appreciate your advice and forwarding the papers to me, they have plenty of information I could adapt for my experiments. 

Best wishes,

Maria