This is a temporary workouround that allows me to go on with the local development with Laravel + Valet + PHP7.4
Today I upgraded my PHP to 7.4.0 version.
I have a MacBook Pro and I use PHP installed via Homebrew.
The usual step are:

brew update
brew upgrade php
composer global update
valet install

After "valet install" I had a problem: Unable to determine linked PHP when parsing '7.4'

This happens because the current version of Valet are not whistelisting PHP 7.4.
I jumped on

code ~/.composer/vendor/laravel/valet/cli/Valet/Brew.php

class Brew
{
    const SUPPORTED_PHP_VERSIONS = [
        'php',
        'php@7.3',
        'php@7.2',
        'php@7.1',
        'php@7.0',
        'php@5.6',
        'php73',
        'php72',
        'php71',
        'php70',
        'php56'
    ];

I added 'php@7.4', line:

class Brew
{
    const SUPPORTED_PHP_VERSIONS = [
        'php',
        'php@7.4',
        'php@7.3',
        'php@7.2',
        'php@7.1',
        'php@7.0',
        'php@5.6',
        'php73',
        'php72',
        'php71',
        'php70',
        'php56'
    ];

I needed to do this workaround, until Valet will include php@7.4
Brew.php

Obviously this is not the official way to have php7.4 but, for me at the moment is working so it allows me to go on with Laravel development on php7.4 via Valet (at least for this weekend).