Install it by:

sudo yum install centos-release-scl
sudo yum install devtoolset-4

The first command installs and enables Software Collections Repository on your CentOS machine. That repository provides the devtoolset package.

Since the package is either not available, or using a different name, the first step is to find which is the case. Use

yum list available |grep gcc
yum list available |grep devtoolset

to find whether you have misspelled the package name, or if gcc is available under a different name. Check using yum info for the package. Its description should show the relevant version information.

According to Red Hat's article What gcc versions are available in Red Hat Enterprise Linux?, gcc 4.8 is available for Red Hat Enterprise 7. If you need gcc 4.8 for RHEL 6, you may have to compile it yourself.

Further reading:

• Installing GCC 4.8.2 on Red Hat Enterprise linux 6.5
• How to Install gcc 4.7.x/4.8.x on CentOS
• install latest gcc on rhel 6 x86_64

While Devtoolset-3 and 4 are located on the Centos mirror site, yum will not be able to find them as they were not included in the repodata on the site. These were probably excluded because they are EOL (end-of-life), and have dependency bugs (Bug 1410152). Try using the older Devtoolset-3 packages here, which do not have the dependency issues:

https://copr.fedorainfracloud.org/coprs/rhscl/devtoolset-3/

If you really want the packages from the centos.org site, I went with wget to download all the RPMs, then recreated a local yum repo. However, you'll see the dependency issues and yum install devtoolset-3 will fail.

$ wget -r -p -e robots=off --directory-prefix="/mnt/local-devtoolset3-repo" --recursive --no-clobber --no-parent http://mirror.centos.org/centos/6/sclo/x86_64/rh/devtoolset-3/
$ sudo yum install createrepo
$ cd /mnt/local-devtoolset3-repo
$ createrepo --database .

You will have to do it the old fashioned way. Get the GCC sources, build it by hand.

Any reason you are averse to doing this? (I can think of a few, but unless you do too, I don't want to add anything that might be confusing).

If you are hesitant to build from sources, I can probably provide some steps.

NOTE: The reason for that is the devtools with 4.9 is currently only available to Red Hat users with a valid license. Wait for a while, and eventually I am sure 4.9 devtools will be made available to CentOS users too like 4.8 is currently.

I am using CentOS 7.9 and I encountered the same problem after following instructions here to install and run gcc 11. I tried launching different versions of gcc and found only devtoolset-9 works, which corresponds to the file devtoolset-9 in /etc/scl/conf/ folder. So I copied devtoolset-9 to devtoolset-11 in the same folder, and gcc 11 gets working.