If you're anything like me, you're bald, mildly concerned about the persistent pain in your right knee, and since childhood have been plagued by recurring nightmares about tidal waves that you secretly fear may be prophetic. You also enjoy running around in Fallout 4 in third-person perspective, but prefer first-person view for combat. You can switch between third- and first-person perspectives manually, of course, and it's not any kind of hassle. However!
There's now a mod that knows how you and I like to play shooters. It's called MGS Aiming, created by modder PeterTran, and when installed it will switch from third-person to first whenever you aim with your weapon sights. Stop aiming, and you snap right back into third-person. I love mods that solve problems you didn't even know you had, and this is a perfect example.
The mod is activated (and deactivated, if you decide you don't like it) with a holotape, and once installed you can enjoy Fallout 4 as it probably already should have been: with a quick snap to first-person when you aim in third-person.