If you are using Shift4 and are get white screens consistently while authorizations/settlement then article can help you. You will get a white screen if your swipe data is in 2 lines. You should double check your MSR (Magentic Swipe Reader) settings. We generally recommend http://www.idtechproducts.com/ and you would need the utility from their website.
Please do a swipe on a notepad, and if the swipe data is coming in 2 lines then you need to change your setting so that the data comes in one line. Following are the steps to do this change.
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Mag Swipe Configuration 

Setting –> Change Reader Setting –> Beep is Off, Coding is in both directions. Track Setting Tab: (Select Track 1 And Track 2) OR You can select (Track 1 OR Track 2)

Track Separator and Terminator is blank. (Remove the /cr from both these fields, the fields should be completely blank)

Send to MSR (Magnetic Swipe Reader) and close the application.