i bought an iphone 4 i reset it but i havent the icloud username and password linked to it, what can i do, plsssss help

Did you buy it used? And is that apple ID not yours? if it is the original owner's then they need to give you the password and unlock it. If they can't or if you can't get that info, get your money back.

What that is is the activation lock and it's mean to be a theft deterrent device. (not accusing by any means, just saying what it is)

Here's more info on what the activation lock is

https://support.apple.com/en-us/HT201365

Skydiver's link has a section on what to do (has a link to another article)