Turbo kicks in when the program, game or stress test that your doing calls for more processing power. If you diable turbo and set the clock multiplier to 40 then the CPU will run at 4ghz all the time.
I always disable turbo when overclocking.
The i5-3470 has a turbo speed of 3.6ghz so that means that you can only have a clock multiplier of x36, you should not be seeing a x40 or a x38 clock multiplier. The only way that you can get to anything higher is to raise the BCLK.