As far as I can see from the Communicate in Greek web site, http://www.communicateingreek.com
there are only three books in the series so I am not sure how Book 4 can be equivalent to A Level? According to the site the full course, i.e. the three books plus associated workbooks and audio material, should take the student to “post intermediate level”.
It is worth noting that the books are completely in Greek. There is not a single word in English anywhere in them. In my opinion, therefore, they are only suitable to be used as part of a taught course with a Greek-speaking teacher to guide you through them. Having said that Jean thinks that they are the best Greek teaching books available. Having had some lessons she can now make use of them herself.
The whole course bought direct from Communicate in Greek would cost about €230 but you can buy it in sections for about €80 each for sections 1 and 2 and about €70 for section 3. I believe these prices include postage. Alternatively you can buy it piece by piece from Hellenic Bookservice, https://www.hellenic-books.com
Stelios writes the “E4 and other mythical trails” on Interkriti and is a true Cretophile; why not support him. Adding up the components gives about £47 for sections 1 and 2 and £40 for section 3 but there would be postage cost on top of this. You can also buy piece by piece from Amazon but the prices seem higher than from Stelios. I cannot guarantee that the editions are exactly the same.