Laravel 4.2 project in Homestead v4.0.0: "Mcrypt PHP extension required."

I've been running Homestead v5 with my Laravel 5.x projects. Today I needed to make an update to an older 4.2 project, tried installing it alongside my other projects but could not escape this error: "Mcrypt PHP extension required." - and google-fu has been failing me. I've seen some similar threads with lots of discussion (like this one: https://laracasts.com/discuss/channels/general-discussion/mcrypt-php-extension-required) so I apologize if it's bad etiquette to open an additional thread like this.

After trying suggestions such as this:

sudo apt-get update
sudo apt-get install mcrypt php7.0-mcrypt
sudo apt-get upgrade

in my Homestead v5 (yes, I ran these on the VM) I didn't get anywhere. I tried installing Homestead v4.0.0 and running this project in there and I am getting the same result. I used Homestead with all of my Laravel 4 projects before, but I can't recall which version it was. I've never experienced this problem before.

Any suggestions how I might setup a separate Homestead box specifically for older Laravel 4 projects to avoid this issue? Or is there something I'm missing getting this to work in Homestead v5?

Thanks for reading.

Edit: When I run sudo apt-get upgrade on either VM, I eventually end up on a config screen for "Configuring grub-pc" where it asks me to make a selection where to install the boot loader. I select "/dev/sda (42949 MB; VBOX_HARDDISK)" for this and it doesn't seem to break anything, but makes me wonder if I should be trying to run apt-get upgrade on the VM at all.