You have some run out on the ash, but its an 8ft board so you might have a straight enough 6ft length.
You could rip the 4 x 4 into 2@ 2 x2, and work down to one growth ring, like a stave bow, then re saw into a backing(with a bandsaw), glue it on to the ipe.
Its a lot of work, but it is a sure fire way of getting a safe backing. The technique was commonly used with ash, and elm by European old time bowyers.