to get the script to act as you want it to, you need the scl command to modify the current bash session rather than spawn a new one.

you can do it this way

#!/usr/bin/env bash

# switch to GCC9 environment for the duration of the script
source scl_source enable devtoolset-9

gcc --version

I ran into the same issue that you are facing and this is how I got it fixed:

1. Just want to be careful, you need to exist the Terminal and open a fresh one to start. This way, you are not under any devtoolset's bash.
2. Go to /opt/rh folder, run command ls -la to see if you have any devtoolset-* folder there. Let's say you have devtoolset-8, proceed step 2.
3. Go to /etc/scl/prefixes folder, if you don't see devtoolset-8 file, you can create a new one as devtoolset-8, and type 1 line: /opt/rh, then save and quit that file.
4. Once you are done, you can call: scl enable devtoolset-8 -- bash w/o any error. Good luck

An alternative of source /opt/rh/devtoolset-4/enable is

source scl_source enable devtoolset-4

The above shell script scl_source is more elegant than using a hard coded path (may be different on another machine). However scl_source does less because /opt/rh/devtoolset-4/enable uses scl_source and other stuff.

To use scl_source you may have to upgrade package scl-utils

yum update scl-utils  # old scl-utils versions miss scl_source

Quick copy-paste

echo 'source scl_source enable devtoolset-4' >> ~/.bashrc
    # Do not forget to change the version ↑

Source code for curious people

An example of scl_source source code:
https://gist.github.com/bkabrda/6435016

The scl_source installed on my Red Hat 7.1

#!/bin/bash

_scl_source_help="Usage: source scl_source <action> [<collection> ...]

Don't use this script outside of SCL scriptlets!

Options:
    -h, --help    display this help and exit"

if [ $# -eq 0 -o $1 = "-h" -o $1 = "--help" ]; then
    echo "$_scl_source_help"
    return 0
fi


if [ -z "$_recursion" ]; then
    _recursion="false"
fi
if [ -z "$_scl_scriptlet_name" ]; then
    # The only allowed action in the case of recursion is the same
    # as was the original
    _scl_scriptlet_name=$1
fi
shift 1

if [ -z "$_scl_dir" ]; then
    # No need to re-define the directory twice
    _scl_dir=/etc/scl/conf
    if [ ! -e $_scl_dir ]; then
        _scl_dir=/etc/scl/prefixes
    fi
fi

for arg in "$@"; do
    _scl_prefix_file=$_scl_dir/$arg
    _scl_prefix=`cat $_scl_prefix_file 2> /dev/null`
    if [ $? -ne 0 ]; then
        echo "Can't read $_scl_prefix_file, $arg is probably not installed."
        return 1
    fi

    # First check if the collection is already in the list
    # of collections to be enabled
    for scl in ${_scls[@]}; do
        if [ $arg == $scl ]; then
            continue 2
        fi
    done

    # Now check if the collection isn't already enabled
    /usr/bin/scl_enabled $arg > /dev/null 2> /dev/null
    if [ $? -ne 0 ]; then
        _scls+=($arg)
        _scl_prefixes+=($_scl_prefix)
    fi;
done

if [ $_recursion == "false" ]; then
    _i=0
    _recursion="true"
    while [ $_i -lt ${#_scls[@]} ]; do
        _scl_scriptlet_path="${_scl_prefixes[$_i]}/${_scls[$_i]}/${_scl_scriptlet_name}"
        source "$_scl_scriptlet_path"
        if [ $? -ne 0 ]; then
            echo "Can't source $_scl_scriptlet_name, skipping."
        else
            export X_SCLS="${_scls[$_i]} $X_SCLS"
        fi;
        _i=$(($_i+1))
    done
    _scls=()
    _scl_prefixes=()
    _scl_scriptlet_name=""
    _recursion="false"
fi

It is because of your settings in .bashrc and/or .bash_profile. scl appends its path to gcc7 before starting your new bash shell, but the path to gcc is default to the old gcc4 again because your .bashrc appends /usr/bin to the path when starting the new bash, and it supersedes the scl settings. The solution is to use the "enable" script directly. That way no further path settings annoy you.