In general, iPhone 4 works flawlessly. As with any other device, you can have certain issues, but most of them can be easily fixed. Some users reported the Wi-Fi problems, when the connection cannot be established, or when the Wi-Fi is very slow. If you have the same problem, here are 5 ways to fix iPhone 4 Wi-Fi connectivity issues, and with several simple tricks, the issue will be gone. If needed, try each solution offered.
See also: 5 Ways To Fix IPhone 4 Won’t Charge
5 ways to fix iPhone 4 Wi-Fi connectivity issues

Source: ourimgs.com
Solution No.1: Airplane mode.
In many cases, the simplest solution of all 5 ways to fix iPhone 4 Wi-Fi connectivity issues, was enough. The only thing you should do, is click on Settings, turn on the Airplane mode, and then turn it off again.
Solution No.2: Restart your modem and router.
The problem you have may not be connected to your phone, but to your other devices. Restart your modem and your router, and see if that will make any difference.
Solution No.3: Reset network settings.
Another simple solution is this: navigate to Settings, click on General, Reset, and choose Reset network settings. It is extremely helpful. However, you should keep in mind that this solution erases all your Bluetooth, VPN and APN settings, as well as the saved Wi-Fi passwords.
Solution No.4: Update your modem drivers.
This may help as well. Type the model and the number of your modem into the browser you are using, and download and install the latest drivers. It may improve the connection, and help you to get rid of the issues you have.
Solution No.5: Reset all settings.
Your preferences will be removed, but on the other hand, your Wi-Fi issue will be most likely solved, which is why you should give it a shot. Go to Settings, click on General, click on Reset, and then reset all Settings.