A question about One

Okay so I was reading in a book about the tone changes to Yi.

It seems that if Yi precedes a forth tone then it becomes a second.

but that if it precedes  a first,second or third tone it becomes a forth tone.

This would appear to mean that the only place you may see Yi with its original tone is when used in a string of numbers, and even then it may be substituted with Yao.  Is this correct?