"Lands, Taxes, and Boats
Saxons can gain land by conquest or inheritance.
Inherited land belongs to the cynn, is divided among living
cynnsmen, and returns to the cynn upon a cynnsman’s
death;"

I'd like to know a little more about how this is carried out.

So say the founder of the line is a proud saxon warrior with 4 hides he helped carve out of Britain. A Hide is valued (by me) at 5L in annual value. The Warrior, who is nearly but not quite a thegn, is nonethless very rich. Upon his death, his 4 sons each recieve 1 hide. What happens next is what I'm confused about.

The 4 brother's will generally not all die at once, so then the land of the first brother to die reverts back to the cynn to be redistributed. The first brother had two sons, as do each of the other brothers. So, how many shares is the hide divided into, and who gets how much?

Personally, I would treat it as simple gavelkind inheritance (all sons inherit equally) rather than the primogeniture inheritance (eldest son inherits everything) practiced by the feudal Cymri in KAP. I'd also read the last sentence there to mean that the land returns to the Cynn if the cynnsman has no inheritors of his own (daughters would inherit equally if there are no sons). This is to make it simple.

If you want to make it hellishly complicated, you could take it at face value:
1. The founder of the Cynn (for instance, the grandfather) conquers some land worth 4 hides.
2. The land is inherited by his sons (say 4, as in your example), so 1 hide each. Assuming that their sons are still minors and hence do not count as cynnsmen.
3. They each have 2 sons, and we assume that they are all adults when Brother A kicks the bucket. This means we are left with 3 sons and 8 grandsons, each of whom count as a cynnsman, and hence the hide would be divided into meager strips of 1/11th hide each. Thus, each grandson would receive 1/11th hide and the brothers would each have 1 hide plus 1/11th of a hide.
4. This same insanely complicated method would happen with each of the sons and grandsons.
5. The fourth generation, great-grandsons, would no longer count the other branches, but just the ones descended from Brother A/B/C/D, since a Cynn is generally determined as having a common grandfather. So it would still be a huge mess.

Just say no, and practice simple gavelkind, is my suggestion.

Another, a bit simplified take, would be to limit it to a generation: all the sons inherit first and only once they are all dead, do their own sons (grandsons) inherit. In this case:
1. The founder of the Cynn (for instance, the grandfather) conquers some land worth 4 hides.
2. The land is inherited by his sons (say 4, as in your example), so 1 hide each. Assuming that their sons are still minors and hence do not count as cynnsmen.
3. They each have 2 sons, and we assume that they are all adults when Brother A kicks the bucket. The grandsons don't count, since the sons are still alive, splitting Brother A's lands for 1.33 hides each.
4. Finally, the eldest surviving son (say D), who had accumulated the full 4 hides, dies. This means the grandsons now inherit equally, 0.5 hides each.
5. Once grandson AA dies, his lands are shared by his brother AB and his cousins (6 in all).
6. Once all the grandsons are dead, their sons (16 great-grandsons) inherit 0.25 hides each. Since they are now second cousins, they are no longer each other's inheritors, but everyone descended from A would be in his branch. So AAA dies and his land would be shared between AAB, ABA and ABB.

I'd still rather use simple gavelkind. :P It works out more or less the same, but without the problems of the constant redivision when someone dies.