OK this is how i flash my firmware.
I have a Windows 8.1 running PC so this is the steps i did.
First you need to download two files so first download below files.
Then you need to install flashtool-0.9.18.3-windows.exe. Just double click and install it and Remember the installing folder (Example - "C:\Flashtool" or "C:\Program Files\Flashtool" like that) and then go to that folder. Inside that folder go to drivers folder and install Flashtool-drivers.exe. Click next check the Flashmode Drivers and Fastboot Drivers (Very important you select those) and go down the menu and select the phone model (Read all well). Then install. If your installation finis without warning errors then you ok. If your installation finis with warning errors then do next step.
Go to PC Settings (Charms Bar) > Change PC Settings > Update And Recovery > Recovery > Restart Now (Advanced Startup)
it will restart to Advance Startup automatically
In Advance Startup -
Troubleshoot > Advanced Settings > Start-up Settings > Restart
After rebooted you will get a screen says. Here to choose options for that we have to use function keys. Now press "F7" to select "Disable Driver Signature Enforcement" .
Now your PC will boot with Disabled Driver Signature Enforcement and you will be able to install Flashtool/Fastboot drivers. so again install the Flashtool-drivers.exe and will finish without errors.
after that again go to falshtool installed folder and go to Firmwares folder and if you see any files already on that firmwares folder delete them. Then copy IS11S_4.0.1.B.0.112_KDDI.ftf file that you download before to firmwares folder. and then run FlashTool.exe (If your PC 64bit then run FlashTool64.exe)
Now on your device go to (Very important to do all the steps below)
01. settings > Application settings > Development > check USB debugging
02. settings > Application settings > uncheck Unknown sources
03. settings > Sony Ericsson > Connectivity > USB connection mode > choose Mass storage mode
Then connect your device to PC using Original USB cable.
Go to flashtool software click on flash icon.
Then chose Flashmode > ok
Then new window will open (Firmware Selector) then you can see the IS11S_4.0.1.B.0.112_KDDI.ftf file already on that.
Now just click on FLASH.!
That's all wait it to finish. and your device will restart automatically. and DONE.!
I couldn't remember where i download those files but i already had those so i upload it for you and anyone who want to download those.
And if this post helps you then be kind enough to click on THANKS for me.!
By - DeadCode - Knowledge is free