Hi rosalind
The second one is mostly a straight detective story, though the siege of Windsor does interrupt it. The 3rd is better than the second. It's set mostly in Chester and Rhuddlan, and we meet Llewellyn ap Ioreth.
I'm not sure about the 4th. It's mostly in Paris and Normandy, and it's rather dark. It's also much longer than the others.
They are of a smilar standard, but, if pressed, I would say that the first one is just about the best.