A 1/5 first five is almost always superior to 1/4 first four.
It can be seen as taking a quarter first four and coupling it with an additional bet to finish exactly fifth. For a 10/1 shot, for example, you’re getting 5/2 first four with the option of taking 20/1 to finish exactly fifth (2/1 coupled). Do you take the optional extra? Almost every time surely. The horse would have to have a very strange profile for it to not be value.
The value is best on longshots. For example, you get 42/1 on a 33/1 poke finishing 5th or 108/1 on a 100/1 shot finishing fifth. These are often two or three times the true price.