#!/bin/bash
# numalpha.sh
alpha=""
numalpha()
{
	if [ $1 -eq 1 ]
	then
		alpha="first"
	fi
	if [ $1 -eq 2 ]
	then
		alpha="second"
	fi
	if [ $1 -eq 3 ]
	then
		alpha="third"
	fi
	if [ $1 -eq 4 ]
	then
		alpha="fourth"
	fi
	# And so on...
}
# Test for integer numbers only.
text=( This is the 1 and only attempt to change the 4 word inside this string. )
count=0
while [ $count -lt ${#text[@]} ]
do
	if [[ ${text[$count]} =~ ^-?[0-9]+$ ]]
	then
		numalpha "${text[$count]}"
		printf "$alpha "
		count=$((count+1))
	fi
	printf "${text[$count]} "
	alpha=""
	count=$((count+1))
done

Last login: Sat Feb 14 22:54:01 on ttys000
AMIGA:barrywalker~> cd Desktop/Code/Shell
AMIGA:barrywalker~/Desktop/Code/Shell> ./numalpha.sh
This is the first and only attempt to change the fourth word inside this string.
AMIGA:barrywalker~/Desktop/Code/Shell> _