In the Book of Souls, clicking the (!) icon near the top right will show you the percentage drops from the Christmas Soul. You have to do a little bit of extra maths to work it out for each specific character, but there is a higher than normal chance of getting a 5* compared with regular Rare souls. Better chances than regular event souls too, but arguably harder to get.
Hmm, the 4* characters might be slightly harder to obtain than the 5*. If I'm doing my numbers right, 12.20% for a 4*, with then a 37.75% chance for the event characters. So 4.6055%. There are 9 of those, so... 0.5117% for a specific 4* character.
4.20% for a 5*, with a high 78.73% of an event character. 3.30666%. Only 5 of those, bringing us to 0.661% for a specific 5*. Hah, I'm more likely to get Scrooge than Coalgiver
Attachment 7308
Or do you mean something more like what they posted in the Arctic Avalanche thread for the Snowflake Soul? I found this rather interesting, and hope they'll post similar charts again in the future. Helps us make more informed decisions, and gives an insight into how they design their various loot boxes.
Attachment 7309