How do i install Ubuntu alongside Windows safely in legacy hardware?

You need enough unpartitioned free space reserved for Ubuntu in order to install with the guided option "Install Alongside Windows".

So do not create an ext4 partition ahead of time. The Ubuntu installer will do that for you.

If you wish to manually partition, you will have to use the "Something Else" option, and manually set up and configure a valid, appropriate partition scheme.

Any time you are installing operating systems and manipulating partitions, there is heightened chance that you make a mistake or something does not go as planned. The chance is never zero for risk of data loss or accident so it would be a good decision to come up with a reliable permanent backup strategy.