There's a lot more to know about dual-booting today because we're in a transition period between the old BIOS-mode boot methods and the new EFI-mode boot methods. A lot of old documentation, and even current replies on forums, assume the former. Most EFI-centric documentation is clearer about this distinction, but some isn't. Thus, if you get the wrong documentation, you'll be led down the wrong path; and even if you read the right documentation, you've still got to figure out how your computer is currently booting (BIOS mode or EFI mode) and match your installation method to the current boot mode. Complicating this is the fact that terminology is confusing -- many people, and even manufacturers, refer to any firmware as a "BIOS," which creates confusion. (BIOS and EFI are two types of firmware with very different boot requirements.) A final confusing twist is that modern EFIs include a feature (the Compatibility Support Module, or CSM) that enables them to boot older BIOS-mode boot loaders. Thus, you may be able to switch boot modes (BIOS vs. EFI) on a single computer. Using one boot mode for one OS and the other for another OS is often possible, but usually difficult.
If your computer shipped with Windows 8 or later pre-installed, it probably boots in EFI mode; however, if you installed Windows yourself, it could use either boot mode.
For more on EFI issues, here's my recommended reading list:
Some of this is Linux-centric, but most of it's general enough to be of interest to anybody.
Note that Windows 10 does not impose any important limit on the number of supported partitions. (I'm sure there is a limit, but it's high enough to not be important.) You may be thinking of the 4-primary-partition limit of MBR, which is relevant for BIOS-mode installations. Under EFI, GPT is used, which has a 128-partition limit by default -- and that limit can be raised, if necessary.
Your description of '"unusable" "freespace"' suggests you might have an MBR disk with four primary partitions. This in turn implies a BIOS-mode installation of Windows. I can't be sure of that, though. You should post back with more details, like cut-and-pasted output of text-mode utilities or a screen shot of the window or dialog box that's showing you an error. (This is general advice; always post the complete and exact error messages you see. In my experience, 9 out of 10 summaries omit the details that are needed for proper diagnosis.) Restoring your system to its original state is also something that would require detailed information on its current state, with one exception: Most PCs today have a recovery/restore function that may be able to do the job. Consult your computer's documentation for details on how to use this tool. Note that it may wipe out your personal data, though!