Connect new 21.5 inch iMac with Retina 4k to a 2014 Macbook Air

I would like to purchase a new iMac, but I can't seem to figure out whether I will be able to connect my 2014 Macbook Air to it and use it as a dual-screen set up. My 2014 Air has a thunderbolt port and the iMac has thunderbolt 3. Does anyone know if these can connect to extend the screen rather than just mirror? I'm looking for more usable screen space. If they can connect, can anyone recommend the adapter and cable I'll need? Thank you in advance!