What happens if WiFi and 4G are turned on at the same time?

4g goes into a sleep when the WiFi is connected.

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When the WiFi is turned on, does 4G turn off automatically?

if they are on at the same time it creates a Paradox and it will cause the fabric of space and time to unravel... lol sorry geek joke.

Yes Wifi takes Priority and your data is turned off.
 
Re: What happens if WiFi and 4G are turned on at the same time

It also depends on what network you use. If you're connected to Wi-Fi then your phone will use the Wi-Fi to send/receive data. It will still remain connected to your carriers data network in the event you need to send/receive MMS, but the connection to your carrier will remain dormant/inactive until it is needed. If you're with T-Mobile, your data connection will be on 3G until transfer is required and then it will switch to 4G if available. I believe on AT&T you remain connected to 4G all the time.

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On my nexus 4 with android 4.3 if the WiFi internet doesn't work it automatically uses data from the 3G.

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On my nexus 4 with android 4.3 if the WiFi internet doesn't work it automatically uses data from the 3G.

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That is how it works. You can turn mobile data off if you don't want it to switch
 
That wasn't the case for my BlackBerry Torch 9800. It favors the mobile network over the Wi-Fi. Went over the limit too many times. Glad I have Android now.

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