The Fourth Doctor Sourcebook was always going to be the most challenging for Cubicle 7 to pull off, at least among the first seven of the series. There are a number of reasons for this, starting with the simple observation that there are far more televised stories for this era than for any other. Each of the first three books spent an average of about four and a half pages per story, filling out the remainder of the 160 pages available with a broader overview and rules.
The 4th Doctor has 41 televised stories: you can immediately see why the arithmetic there is going to pose a problem.
Many multi-volume guidebooks of Doctor Who history have addressed this issue by splitting the era between two volumes. Cubicle 7, however, have taken the approach of just giving us more. This volume is a full 256 pages, over half again the length of the others. It's also noticeably jam-packed with content to an even greater extent than in the three we've already seen.
However, sheer length alone is not the only problem with bringing the 4th Doctor era to life. The audience is likely to be demanding, since this era is widely acknowledged as the best and most popular of the entire classic run. In polls to determine people's "favourite Doctor", only the 10th regularly offers up any challenge (and which of the two comes out on top largely depends on the demographics of your poll respondents). Looking specifically at the classic era, polls of "favourite DW stories" are dominated by Tom Baker, with over half of the entries in any top ten typically coming from this period.