chujiten: 横綱ははたき込みを食って土俵に四つんばいになった.　 The Yokozuna was clapped on the back and went down in the ring on all fours.
prog: 彼は倒れて四つんばいになった  He dropped [fell] to his hands and knees.

I think the その姿勢 refers to the state of being on all fours (i.e. without crawling). "sprawling" suggests that the arms and legs are outstretched. I don't think it's a good fit.

@@ -16,2 +16,2 @@
-<gloss>getting on one's hands and knees</gloss>
-<gloss>sprawling (on the floor)</gloss>
+<gloss>being on all fours</gloss>
+<gloss>being on one's hands and knees</gloss>

I agree "falling flat" doesn't really fit with the "四つん". Maybe this?

@@ -17,3 +16,0 @@
-</sense>
-<sense>
-<pos>&n;</pos>
@@ -21 +17,0 @@
-<gloss>falling flat</gloss>

I don't think it's 2 senses at all, except if you regard また、その姿勢 (daijs) as something separate. I think GG5's translations only make sense if you take into account the (倒れて), and even then "falling flat" is a weird choice.

GG5: 四つん這いになる get [drop down] on all fours; 〔倒れて〕 fall flat; be sent sprawling (on the floor)
中辞典: つんばいになる get [drop] down on all fours
･四つんばいになって 《go, crawl》 on all fours; on (one's) hands and knees

It's sort-of two senses. Maybe this works. If approved 四つ這い should be aligned.

@@ -16,0 +17,4 @@
+</sense>
+<sense>
+<pos>&n;</pos>
+<gloss>sprawling (on the floor)</gloss>