Why not multisite:

• Multisite installations are difficult to maintain compared to single site installations.

• Many useful plugins don’t work properly on multisite installations.

• There may be SEO concern for such short web sites with basically just one page (thank you page doesn’t count) that looks very similar from the names. Multisite won’t solve this probable issue.

I have assumed they have similar content from your example domain names, however, if the content of the four sites are very very different, then form an SEO point of view, having separate sites with a Multisite installation may be better.

Why not single site:

• Managing multiple languages may not be easy in a single site installation if you haven’t done that before.

As @Mark suggested in the comment, if you have different language contents in those sites (again just assumption based on example domain names), merging them into a single site may not be easy. You are encouraged to use plugins such as Polylang to manage multiple languages. There are thousands of multilingual sites out there running perfectly fine with WordPress. However, if it turns out to be difficult, then you may consider setting up Multisite or even keeping it as it is.

Leave them as they are?

If the following conditions are true, then may be it’s better to just leave them as they are:

• Managing these multiple separate installations are not too difficult for you.

• You may have plugins that are not well tested in multisite installations.

• You have multilingual content in different sites and you have no experience with WordPress multilingual setup.

Steps for single site installation

If you decide to go for single site installation, or at least want to test it, you may follow the steps below:

1. Keep any one site (for example dogsrule.com) as the main site.

2. Then make the other three sites point to the main site’s web root directory.

3. The main site’s home page remains as is & then you create three more pages for the other three sites (for example: dogsrule.com/cn, dogsrule.com/dogs-rule-cats-drool and dogsrule.com/au).

4. Then you may use server rewrite rules to redirect other three domains to the corresponding pages of the main site.

For example, if your web server is Apache, then in your main site’s .htaccess file you may use the following CODE (according to the above pages):

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

# redirect rules for the other three domains
RewriteCond %{HTTP_HOST} ^dogsrule\.cn$
RewriteRule .            http://dogsrule.com/cn [R=301,L]
RewriteCond %{HTTP_HOST} ^dogsrulecatsdrool\.co\.uk$
RewriteRule .            http://dogsrule.com/dogs-rule-cats-drool [R=301,L]
RewriteCond %{HTTP_HOST} ^dogsrule\.au$
RewriteRule .            http://dogsrule.com/au [R=301,L]

RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

After this you’ll have just one site with four main pages (including the home page) and four thank you pages and your other domains will correctly redirect to the corresponding pages.

Note: You may remove the other three sites after setting up the above, but don’t delete the domains. Even though, effectively you’ll only have one site after this, however, if you delete those domains, old bookmarks and back links will not work.