I think I've finally worked out what is going on in Ruth 4. I'll make a note here as a place where I can come and find this again when I need it. Do comment below if I've missed something.
There are 3 Old Testament laws in play here.
Leviticus 25:23-28 says that, because all the land is really God's, should someone sell part of their land to alleviate their poverty, the buyer cannot regard it as theirs absolutely. A relative of the person they bought it from must be allowed to redeem, or buy back, that land, and the price for that is to be calculated fairly.
Numbers 27:1-11 says that, should a man die without a son, his nearest relative is to inherit his land so that his land does not leave the family. Should he have a daughter, his daughter is the first contender for this.
Deuteronomy 25:5-10 says that, should a man die without a son, his brother is to marry his widow. Their first son will be reckoned as the son of his brother.
Put them together. Naomi is a widow; her and her former husband now have no sons. However, Ruth, as daughter-in-law could arguably (Numbers 27) be allowed to inherit the family land. Daughters-in-law aren't specifically mentioned in Numbers 27, however, so really this needs a near relative to redeem the land from the person who bought it (Leviticus 25). Here's the catch, however. Ruth's husband died when they had no sons. So the minute someone steps up to the post and lays claim to being Ruth's nearest relative, Deuteronomy 25 says that they have a duty to marry Ruth as well. The nearer-than-Boaz relative wasn't willing to do this, so he was happy for Boaz to stake his claim as Ruth's nearest relative.
If anyone wishes to add anything, please leave a comment below.