10/10 condition original leather back with NFC + Wireless charging support costs 4000 at max. Mostly you can get it in 2000-3000 very easily from local market.
No idea about specific place in Lahore, but Hafeez Centre is a good place to find that leather back.
Btw, your G4 is less than one month used, as you got it 100% original brand new from your relative shop and you never mentioned it in ad.