iPhone 4 Activation Lock

just it does not unlock anymore.

Have you forgotten your passcode?

If so, see to unlock your iPhone, follow the steps described in the Apple article-> If you forgot your iPhone passcode -> If you forgot your iPhone passcode - Apple Support

If you have bought 2nd hand iPad, you must contact the seller (hopefully it is the previous owner), otherwise you have been scammed.

Each Apple device is linked/associated with an Apple ID of the person who did the setup.

Activation Lock prevents anyone who is not the owner from using this iPad.

To unlock the Pad, you need the Apple ID and password of the previous owner, that were used during set up.

So you have to contact the previous owner.

The previous owner has to read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch -> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Then the previous owner has to remove the iPad from his account, see this Apple article:

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to find where you're signed in - Apple Support

Then you can sign in with your own Apple ID.

See the following Apple articles:

Activation Lock for iPhone, iPad, and iPod touch

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

How to remove Activation Lock -> How to remove Activation Lock – Apple Support (UK)

• If you need help removing Activation Lock and have proof of purchase documentation, you can start an Activation Lock support request.

Read this before you buy a newer iPad.

You can also contact Apple support for help, -> Get Support (apple.com)

You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.

If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.

Once Apple receive your proof of ownership, Apple will further review your request and provide an update.

See also -> Keep your receipt after you buy an Apple device

Keep your receipt after you buy an Apple device – Apple Support (UK)

Ask your money back if possible, perhaps you have bought a stolen iPhone.

If you can't get in touch with the previous owner, you can't use the iPhone.

At this point, it doesn't matter. The iPhone 4 does not support 4G networks, and all carriers shut down 3G networks at the end of last year. In other words, you phone is obsolete and you'll need to purchase a new, or newer phone.

Depending on which part of the world, which country, there are still countries where one can use 3G, such as:

UK

https://www.three.co.uk/support/network-and-coverage/our-plans-to-switch-off-3g#:~:text=When%20will%20the%203G%20switch,don%27t%20already%20have%20one.

UAE

https://www.etisalat.ae/en/c/generic/goodbye-2g.html#:~:text=Will%20there%20be%20only%203G,coverage%20and%205G%20where%20available

in the Philippines

https://www.opensignal.com/2021/06/16/how-to-accelerate-3G-switch-off-to-improve-the-philippines-mobile-experience#:~:text=Much%20of%20this%20improvement%20has,their%20use%20of%20wireless%20spectrum

I did almost write "in the U.S.", but made the presumption the OP is in the U.S., so I left it out.