I am traveling from the UK to Delhi and I am bringing 4 phones with me + mine the rest are for friends. These phones are all used so will this be an issue in bringing them through customs?
As per the Indian customs rules:
There is a limit on the number of laptops you can bring back, but none for mobile phones. That would fall under the standard limit of INR 35,000 / INR 15,000 depending on how many days you were away from India as shown in the table above.
You haven't mentioned the make of the phones. Ideally, you'll need receipts to prove what the original purchase price was so that depreciation can be calculated, or "used" price was. I'm guessing they'll bother if, say, it was an iPhone as compared to a brick Nokia phone.