Can I have rectangular grid workspace in Ubuntu 18.04?

The option to install GNOME extensions using GNOME Tweaks is removed from last few versions of the application. But you can simply install and manage extensions from their official websites. To do that first install the chrome-gnome-shell package by running the following command in Terminal

sudo apt install chrome-gnome-shell

Then visit https://extensions.gnome.org/ using Firefox or Google Chrome, the website should ask you to install one browser add-on/extension. Install it.

Then go to the extension's homepage, you'll be able to install the extension just by toggling on the button at the top-right:

Alternatively, you can manually install extensions following this answer.

The Workspace Matrix extension is better than Workspace Grid in my experience. It's based on Workspace Grid, but fixed to work properly on Ubuntu 18, including thumbnails and some other weird kinks that Workspace Grid has (the grid was randomly changing size as I switched between workspaces, for example):

https://github.com/mzur/gnome-shell-wsmatrix

Install it by searching the Ubuntu Software Center for Workspace Matrix:

TL;DR: Yes, you can.

This is the alternative answer in case you're too lazy to install that GNOME extension.

You can install Unity desktop if you don't mind downloading and using other than GNOME desktop. It's pulled back to universe repository (unlike 16.04 which is in main), which Canonical doesn't maintain anymore.

Simply type this command in terminal:

$ sudo apt install ubuntu-unity-desktop unity-tweak-tool

You might be prompted with choices of default desktop manager. Pick either gdm or lightdm.

Log off/reboot and select Unity when logging in. Then, open Unity Tweak Tool and click Workspace Settings.

Here, you can configure it to have 2 by 2 workspaces.