I think you are close. You are just mixing oauth and service account.

With your UA analytics code you are using the Google analytics reporting api / google analytics management api to request access of the user to access their Google analytics accounts. You are currently using Oauth2 to request access of a user to access "their" google analytics accounts. These are accounts that they control.

With your GA4 accounts you are connecting to the google analytics admin api though you are currently using a service account.

Service accounts must be pre authorized. They are intended for use with accounts that the developer owns. You need to swtich it to use Oauth2 and not a service account. This way the users will be authenticated to their own account.

Google analytics admin Oauth2

Here is a sample with Oauth2.

<?php

// composer composer require google/analytics-admin

require 'vendor/autoload.php';
use Google\Client;

use Google\Analytics\Admin\V1alpha\AnalyticsAdminServiceClient;

putenv('GOOGLE_APPLICATION_CREDENTIALS=C:\YouTube\dev\credentials.json');  // Installed app credentials.

$credentials =  getenv('GOOGLE_APPLICATION_CREDENTIALS');

$myfile = file_get_contents($credentials, "r") ;

$clientObj = json_decode($myfile);

$client = getClient();

$tokenResponse = $client->getAccessToken();

print_r($tokenResponse);
print_r($tokenResponse["access_token"]);


$service = new AnalyticsAdminServiceClient( [
    'credentials' => Google\ApiCore\CredentialsWrapper::build( [
        'scopes'  => [
            'https://www.googleapis.com/auth/analytics',
            'openid',
            'https://www.googleapis.com/auth/analytics.readonly',
        ],
        'keyFile' => [
            'type'          => 'authorized_user',
            'client_id'     => $clientObj->installed->client_id,
            'client_secret' => $clientObj->installed->client_secret,
            'refresh_token' => $tokenResponse["refresh_token"]
        ],
    ] ),
] );

$accounts = $service->listAccounts();

foreach ($accounts as $account) {
    print 'Found account: ' . $account->getName() . PHP_EOL;
}


function getClient()
{
    $client = new Client();
    $client->setApplicationName('Google analytics admin beta Oauth2');
    $client->setScopes('https://www.googleapis.com/auth/analytics.readonly');
    $client->setAuthConfig(getenv('GOOGLE_APPLICATION_CREDENTIALS'));
    $client->setAccessType('offline');

    // Load previously authorized token from a file, if it exists.
    // The file token.json stores the user's access and refresh tokens, and is
    // created automatically when the authorization flow completes for the first
    // time.
    $tokenPath = 'tokenAdmin.json';
    if (file_exists($tokenPath)) {
        $accessToken = json_decode(file_get_contents($tokenPath), true);
        $client->setAccessToken($accessToken);
    }

    // If there is no previous token or it's expired.
    if ($client->isAccessTokenExpired()) {
        // Refresh the token if possible, else fetch a new one.
        if ($client->getRefreshToken()) {
            $client->fetchAccessTokenWithRefreshToken($client->getRefreshToken());
        } else {
            // Request authorization from the user.
            $authUrl = $client->createAuthUrl();
            printf("Open the following link in your browser:\n%s\n", $authUrl);
            print 'Enter verification code: ';
            $authCode = trim(fgets(STDIN));

            // Exchange authorization code for an access token.
            $accessToken = $client->fetchAccessTokenWithAuthCode($authCode);
            $client->setAccessToken($accessToken);

            // Check to see if there was an error.
            if (array_key_exists('error', $accessToken)) {
                throw new Exception(join(', ', $accessToken));
            }
        }
        // Save the token to a file.
        if (!file_exists(dirname($tokenPath))) {
            mkdir(dirname($tokenPath), 0700, true);
        }
        file_put_contents($tokenPath, json_encode($client->getAccessToken()));
    }
    return $client;
}

Note this is a console app, not a web app. You may have to tweek it to work with web but you should just be able to copy your UA auth code and dump the tokens in as i have shown here.

Code shamelessly copied from Simple How to Integrate php with Google analytics admin api.

To fetch Account Summaries using Access Token, you can use Google APIs Client Library for PHP

Here is the working code

<?php
require_once 'vendor/autoload.php';

use Google\Client;
use Google\Service\GoogleAnalyticsAdmin;

$access_token = 'ya29.xxxxxxx';

// Init Google Client and Set Access Token
$client = new Client();
$client->setAccessToken($access_token);

// GA Admin Service
$service = new GoogleAnalyticsAdmin($client);

// Fetch Account Summaries
$summary = $service->accountSummaries->listAccountSummaries();

// Output
print_r($summary->toSimpleObject());

Output

stdClass Object
(
    [accountSummaries] => Array
        (
            [0] => stdClass Object
                (
                    [account] => accounts/XXXXXXX
                    [displayName] => XXXXXXX
                    [name] => accountSummaries/XXXXXXX
                    [propertySummaries] => Array
                        (
                            [0] => stdClass Object
                                (
                                    [displayName] => XXXXXXX GA4 Property
                                    [parent] => accounts/XXXXXXX
                                    [property] => properties/XXXXXXX
                                    [propertyType] => PROPERTY_TYPE_ORDINARY
                                )

                        )

                )

            [1] => stdClass Object
                (
                    [account] => accounts/XXXXXX
                    [displayName] => XXXXXXX accounts
                    [name] => accountSummaries/XXXXXX
                )

        )

)