I don't know any of the details, but dram is basically a whole lot of bits and and each bit is one capacitor and one transistor.

In the diagram for 8F Squared, there's a capacitor at every other junction between word lines and bit lines. If the lines (and inter line spacing) are 2F wide, you have one bit in a 2Fx4F rectangle, or one bit per 8F squared.

In the diagram for 6F Squared, there's a capacitor at every bit line on every other word line. If the lines (and inter line spacing) are 2F wide, you have one bits in each 2Fx3F Rectangle, or one bit per 6F squared. (I guess the word lines without bits are needed for spacing or to drive the transistor or maybe it's just illustration and doesn't mean much)

In the diagram for 4F Squared, there's a transistor at every bit line on every word line. Now you have a bit in every 2Fx2F square, or one bit per 4F Squared.

If you can make dram in all three styles with the same F, 4F squared is twice the density as 8F squared. If the F doesn't hold constant, it's squishier, of course.