You can run other stress testing programs like OCCT, IntelBurnTest, LinX and HCI memtest to ensure that you have covered more areas that might cause instability. Even then, you're still not guaranteed 100% stability but your chances are only higher. Only Intel knows whether the CPU is 100% stable overclocked but they definitely wouldn't publish their custom written stress testing software to the public.

4.2GHz is pretty mild, you shouldn't encounter any issues. If there is any instability and it BSODs, write down the BSOD code and check for it. Most of the time it needs you to increase the voltage for more stability.

My usual procedure for testing an OCed system:
1.)Set Overclock Setting to the desired setting
2.)Use IBT 3 Loops,set to (Total memory GB-1GB,eg:4GB-1GB=3GB)
3.)Make sure it pass
4.)Than lower vcore
5.)Final setting run 20 loops IBT with same memory setting as 3 loops

If this pass,the system is quite stable already