iphone 4s< update and find my iphone
If you update the phone, you might be prompted to reenter the password that you do not know. 

However, if the update goes bad due to file system corruption on the device you will not have the option to erase the device and start over. 

For that reason I would recommend not doing anything until you can get control of the device or you have a budget to replace it should you lose control of the secondhand and activation locked phone.