What drivers should I download for my lenovo flex 4 laptop that has a different hard drive?

Maybe a compatibility issue if no 8.1 drivers initially. I would use driverbooster or similar and see it able find drivers auto, can't advise really as the chipset etc, could work… May need go back to windows 10

There's no god

Windows 10 drivers do not work on Windows 8. They will refuse to install most of the time. If they do, they will make your computer perform worse then if they are not installed.

If there are no win 10 drivers, you can continue to run the laptop on the generic drivers in Windows 8 (that is what it is doing now.) But you will not get the best performance.

You would be smarter to upgrade the system to Windows 10 and install the correct drivers.

You would be smartEST to put the original hard drive back in, copy over your data and reinstall any programs you need on the new drive. If you move over the old drive, you bring all the errors, abandoned files, orphaned registry settings, bad sectors, etc of the old hard drive. Plus your drive will fail sooner if you continue to use the old drive because it is already "old". (Average life for a hard drive is 4 years.)

Plus you will be bringing over settings, drivers, etc meant for your old hardware. Those will prevent maximum performance on your new PC.

Are you expecting it to boot up and run? Some may some may not you can't just switch hard drives and expect it to work but windows 8.1 should install the needed drivers if you can get on the internet