Technically I think there are six. The later ones are definitely inferior to the earlier ones (and have some questionable writing decisions made about their main female character) but I feel like the fifth one is entirely worth it for a sequence in which Charles gets very gratifyingly whumped for a change. (I wrote about it here - contains fairly large spoilers.) I think it's nowhere near equal to Francis at his best, but I've definitely read books by actual Dick Francis that were worse.