Is there a way to use fingerprint only for entering the iTunes & AppStore password? I'd like to use my fingerprint instead of entering the iTunes password via keyboard.  However, for that to happen, I have to define a 4 digit passcode.

And despite the fact that on the Touch ID and Passcode page, i've deselected iPhone Unlock, it still makes me enter the password.

How can I use the fingerprint exclusively for entering iTunes and App Store credentials?
You can only use Touch ID if you have a passcode set. Turning off passcode will disable all Touch ID features. (switching on just "Use Touch ID for iTunes &amp; App Store" will enable the passcode globally).

Either you'll have to jailbreak your device and resort to tweaks such as CleverPin (disables passcode when connected to your home network), or use Touch ID/Passcode to unlock your phone.

If you would like to know more about jailbreaking, I suggest you check out this FAQ  (there are lots of misconceptions about jailbreaking) I will also gladly answer any questions you may have, and definitely head over to the /r/jailbreak subreddit for additional help and support. A lot of the tweak developers are active on the subreddit, it's also a good place to request certain tweaks.