I've read many many comments complaining about the change of cast, which is as I could conclude, the main reason for disliking season 3 and 4. I've tried watching the s01e01 when it came out, but as someone not familiar at all with the British crown and their history, I found it too hard to follow and gave up immediately. Then they have casted 2 actresses I admire - Olivia Colman and Helena Bonham Carter so I decided to give it a try once again just because of them, hoping that I could understand it better now that Charles and Diana are in the timeline, because that's something I remember as a kid. And I liked it a lot. Cast is tremendous, story well done, I got acquainted with the characters and after finishing season 3 and 4, I've decided to try again with the season 1. And I liked it too, although still a bit less than 3&4. Claire Foy and Vanessa Kirby are also amazing and yes, there's a bit of a change in tone in HBC portrayal but it's not huge and unbelievable. Except Olivia looks older than her age, there were no discrepancies between her and Claire's portrayal of queen Elizabeth. Except eye color, but it's silly to mention or complain about it. I'm looking forward to season 2 and 5 and 6. But I dislike casting Elizabeth Debicki as older Diana, I always found her acting plain, lifeless and (n)one-dimensional. But I hope they know what they're doing, that she wasn't chosen for being slim blonde with slightest physical similarity to Lady Di and that she won't ruin yet another movie/show.
Review of The Crown
