Sorta continuation of this, here's some observations on how the bsd anime adaptation rate (average of number of chapters adapted per episode) changed every season:

Season 1: 10 plot episodes + 2 ln adaptation. Adapts up to chapter 16. Average of chapters adapted per episode: 1.60

Season 2: 8 plot episodes + 4 ln adaptation. Adapts chapters 17-37 (21 chapters). Average of chapters adapted per episode: 2.63 (+1.03)

Season 3: 9 plot episodes + 3 ln adaptation. Adapts chapters 38-39 & 41-53 (15 chapters). Average of chapters adapted per episode: 1.67 (-0.96)

Season 4: 10 plot episodes + 3 ln adaptation. Adapts chapters 54-77 (24 chapters). Average of chapters adapted per episode: 2.40 (+0.73)

※ I opted out of counting the Hitori Ayumi OVA this time around, reasoning that the factors that play into conditioning the season episodes adaptation rate wouldn't have influenced the OVA given its nature of stand-alone episode.

Seasons 1-3 adaptation rate: 27 plot episodes, 52 adapted chapters. Average of chapters adapted per episode: 1.93

Season 4 adaptation rate compared to the average of all the previous seasons: 2.40 (+0.47)

Seasons 1-4 overall adaptation rate: 37 plot episodes, 76 adapted chapters. Total average of chapters adapted per episode: 2.05

That being said, let's try to guess how season 5 might play out taking in the previous four seasons adaptation rate of 2.05 as standard. Here's the fun thing: if season 5 is just 12 / 13 episodes of plot adaptation, that would mean adapting up to chapter 102 / 104. Coincidentally, chapter 104 (fukufuku flashback) is also the furthest in the manga plot that was shown in the last season 5 trailer. Feel free to draw your own conclusions, but as of now a new light novel adaptation seems positively unlikely and a Stormbringer adpatation basically impossible to make fit.