Hi,
I heard this apologetic a few times that apparently 'answers' this question.
The explanation suggests that these creatures do not have four feet, they could have any number of feet, but 'going on all fours' is a term that is used to described a person crawling on the floor. We have four limbs so when we crawl we go on 'all fours' and some apologists claim that the authors of the Bible used this phrase to get across an image, their audience would know what was meant by going on four legs.
One of the major problems with this is that the saying 'going on all fours' comes from Jacobean times and appears in the KJV, and it is being used to explain what Hebrews writing 2000 years before the KJV were actually meaning.
The legs above their feet is meant to be a reference to the way grasshoppers move their feet!
It must be a nightmare being a literalist, the cognitive dissonance that they display is embarrassing.
Brian