Hi All
We are looking at having a few days on mainland Greece (will already have had a couple of days in Athens) and am after recommendations on what to see before heading out to some of the islands. Unsure of exact number of free days - perhaps 3 or 4. Any ideas are most welcome. Thank you

In only 3-4 days, you will not see much of mainland Greece.
Most Islands are best reached from Athens airport, Piraeus, Rafina or Lavrio, others from Killini, Igoumenitsa, Gythion, Volos, Kavala or Alexandroupoli

There's so much to see on mainland Greece -- so many wonderful things! -- that it's hard to suggest anything, particularly in the absence of information about your interests. Instead, I'll suggest you take a look at a good guidebook, perhaps the Rough Guide, which is very good for Greece. And you might take a look at my trip report. The section on mainland Greece begins with post # 125.
With Gratitude for a Glorious Solo Month in Greece

An idea would be to take the KTEL bus or the train and go to the Peloponnese (the north-east side). Visit Nafplio for example and then take the boat to the islands nearby (like Hydra, Poros, Spetses) and from there island hop to other islands. We've done something similar (but with our own car) and it was very nice. We used the ferry hopper map to find places that combine boat routes and nice cities in the Peloponnese, you can see that here https://www.ferryhopper.com/en/maps/ferries-map it has helped us quite a lot.

With only 3-4 days, I would stay in Nafplio and maybe do a short day trip or two from there, though I could certainly entertain myself staying in town for that amount of time if it was my first visit.