When I teach I try to get my students to walk to the foul line as natural as possible as this is the easiest thing to duplicate.
And yes if you are currently taking four steps and are moving back to five, basically take a full step back. But either way try and make your steps as natural as possible.
The methodology between 4 & 5 steps is timing. Most bowlers I work with tend to be a full step out of time due to the fact that they take 4 steps yet do not move the ball until the second step. This is probably the biggest fault I see the average bowler have.
Some bowlers are able to move the ball & step at the same time. They can manage a 4 step approach and be close to in time.
Some bowlers need the extra step to create movement or motion, comfortably to get the ball moving on the second step. Some logic is 5 steps allows an extra step to screw up but the flip side is that 4 steps is too stringent.
The views and opinions expressed by myself are solely those of mine and NO one else, nor are they affiliated with anyone else.