round and flat

It really depends on what effect you want. As you wrote four strand braids can get several different shapes. There are also several different techniques on the flat one, some that naturally twists and some that are just flat. 

The two kinds I use most often is round (for sleep braid) and flat (that doesn't twist). I found the round to be a bit more complicated to learn but I really like the look of it and it also doesn't seem to give very much wave (my hair doesn't look very good with braid waves). I'll try to explain how I do them:

Round:
Divide the hair into four equal strands. Take the right most strand and move to the left over two strands and then back (to the right) under one.

Then take the left most strand and move it to the right over two strands and then back (to the left) under one.

Continue like this.

ETA: It's, off course, also possible to do this braid dutch style. You just change it to under two back over one instead.

Flat:
Divide the hair into four equal strands. Take the right most and weave it, to the left, under one strand and over the next.

Take the left most strand and weave it, to the right, over one strand and under the next.

Continue like this.

I hope you can understand  my explanations and please exuse my poor paint drawing skills.

My hair is thin, tail bone length with a very heavy taper and I can do both kinds all the way down without too aching arms. lol

cross made of my own and my mothers hair

1a/bFi/ii

16/38/38
Maintaining at tail bone for 2009 trying to get a thicker hemline

Routine:
Daily scalp wash with CV bars followed by ACV since.
Treatment with coconut oil, aloe vera gel and deep conditioner once or twice per week.
Oiling with coconut oil when needed.

Horn combs and BBB brush.