I would swap your CON stat for DEX, helps your AC and boosts quite a few proficiencies.
As a fiendlock, you can get more temp HP from killing enemies and Armor of Agathys works well in the interim as well.
As for the background, why not follow the PHB's suggestion to create one that you like? On page 125-126 on the PHB, there's an entry for customizing your own background.
"To customize a background, you can replace one feature with any other one, choose any two skills, and choose a total of two tool proficiencies or languages from the sample backgrounds."
Just pick the two skills you want, 2 tool proficiencies/or languages or mix them up, and pick one feature. Once you look thru it, a background with those skills will naturally appear.
Stealth and Deception with Thieves Tools and Forgery Kit, that's a combination of the Charlatan and Urchin and can be reskinned as a Spy or something.
Feature, Criminal Contact reskinned as Information Network.
Before you go Book of Shadows, is there another wizard in the Party? And check with your DM how likely you are going to be able to find scrolls and teachers to learn rituals.
No point making a ritual caster without the necessary teachers, keeping in mind Druids and Cleric only spells may not readily have scrolls lying around being Divine and all.
Pact of Chain will give you a lot of Creative uses. Also your familiar may be able to share it's magical resistances with you (if your DM is kind).
If you're taking Eldritch Blast upgrades in the Invocations, you may want to avoid the Pact of the Blade as well, too many invocations required.
You can compensate using Armor of Agathys to keep you safe in melee range and blast from afar, healing up THP everytime you down a Hero via Fiend Pact.
Lastly, you mentioned you start at level 4, that's an Ability Score Increase or Feat.
CHA or DEX are good options to upgrade, one being more offensive, the other being more defensive.
Lucky can also be considered and will essentially negate any 'Disadvantage' on your part or cancel a Critical Hit on an enemies part.