Uninstall gcc-5.4.0 to run gcc-4.8 as default

Indeed, there is no "uninstall" command when installing from source, unless the developer has decided to put one in. (i.e., it's not impossible for a developer to create a script that undoes everything)

Your only option is to go into /usr/local/bin/ and remove each file yourself. But which ones to remove?

One solution is to reinstall 5.4, but specify a new directory that isn't /usr/local/bin/. This will give you a list of files that 5.4 installed. Then, using the list of files, go into /usr/local/bin/ and remove them manually. Note that it probably installed libraries and documentation into other directories, so to remove it completely, you will have to do more than just /usr/local/bin/.

An alternative is to not do an uninstallation and just install gcc 4-8 on top of 5.4. It's the same program, but just a different version. So gcc version 4.8 will overwrite 5.4. The "downside" is that you might have 5.4 files lying around, but they will just occupy (relatively little) space. It'll solve your problem and you can move ahead with what you want to do.

Of the two options, I'd pick the second one. The 5.4 files that remain won't affect your system. Relative to images, video, or almost any type of data, the space they occupy is fairly little (i.e., compiler, libraries, documentation is probably small). In the future, you should install it in another directory and add symbolic links manually. update-alternatives would do that for you if it's a package installed compiler; but for a compiler installed from source, that isn't an option.

(Aside: Ubuntu 14.04 has reached end of life, I think. You might consider upgrading some day... If so, then what /usr/local/bin/ looks like won't matter anymore!)

I advise to use update-alternatives command as described in the answer How to choose the default gcc and g++ version?

Just create a symbolic link to gcc-4.8

cd /usr/bin
sudo rm gcc
sudo ln -s gcc-4.8 gcc

now try

gcc --version

And see if you got the desired result