Securing my PS4 data before lending it

Okay time to hit the list here:

• He would be able to play all of your games if he is using your account or not.
• If you want to be really safe with your saves just get yourself a flash drive and back up all your saves onto it. Link on how to do this. You can easily replace any new saves with your old ones saved onto your thumb drive.
• Simply remove your credit card information from your Playstation Network account. Link on how to do that as well.
• If your brother-in-law is using your PSN account and those account are already logged into your apps like Netflix, he should be able to use those without having to log in at all.

Hope these help.

For each of your points:

• Playing your games is a non-issue, assuming your account has that console listed as your primary console. All users will be able to use the content that belongs to the account with the console listed as its primary. As for discs, see this video for how to share. (Spoiler: you just have to give them the disc)

• Preventing someone from using your saves is a little tricky, as anyone who decides to log in as you will have full access to your save files. You could save them on a USB, as explained here. However, your saves will still be in the cloud if you're a Playstation Plus member, so they may still mess with that version of them. Having back-up versions will solve this problem, though.

• Simply remove the payment method from your account for the time being. Here is a tutorial for that.

• To access your Netflix, Hulu, Crunchyroll, Funimation, etc. accounts, your brother-in-law and/or his friends will need to be on your account. Based on my advice for the other three points, this shouldn't be an issue. If it is an issue, however, then the only way for you to allow them access off of your account is to log in on their account(s) as yourself.