turbo boost wont turn off

Are you using balanced or performance power plan? If you're on performance change it to balanced.

Your CPU is only supposed to turbo to 4.0 GHz normally on a single core. If it's stopping at 3.8, that's likely because you are loading more than a single core.

Performance mode will have your turbo constantly on. If you want to go back to stock 4Ghz and 4.4Ghz turbo, leave it on balanced and put your settings in the bios back to default. If you want to run at turbo speed full constantly change the power plan to performance. These newer processors are different than the old school ones; on my i7 960 I'd disable turbo boost and increase the BLCK and be running at 4.1Ghz, but on my 4770K and 5930 I leave turbo on, the o/c is applied with turbo boost basically as far as I can tell; i.e. I sync all cores at x45 and leave turbo on in the bios, then turn performance mode on in Windows and then I'm running at 4.5Ghz all the time; or I use balanced mode and hit 4.5Ghz when the computer needs it.

Only the k version of the 4790 clocks at 4 GHz - 4.4 GHz turbo.

If more than 1 core of your CPU is loaded, it will not boost to 4.0 GHz. If you are benchmarking, unless the benchmark is specifically designed to only load a single thread, I would not expect to hit 4.0 GHz.

Below is the page Intel posts for their Turbo boost used on the 4790. Basically, 100% for the 4790 that has the Turbo Boost feature turned on is somewhere between 3.6 GHz and 4 GHz, not 4 GHz. Between 3.6 - 4, you are subject to the throttling of Turbo Boost, based on loading of the processor. If you want a constant 4 GHz, you're going to have to disable the Turbo Boost or work within it's limits to make it happy enough that it doesn't force throttling.

Intel® Turbo Boost Technology 2.0

"Intel® Turbo Boost Technology 2.01 accelerates processor and graphics performance for peak loads, automatically allowing processor cores to run faster than the rated operating frequency if they’re operating below power, current, and temperature specification limits. Whether the processor enters into Intel Turbo Boost Technology 2.0 and the amount of time the processor spends in that state depends on the workload and operating environment.

Maximum turbo frequency indicates the highest possible frequency achievable when conditions allow the processor to enter turbo mode. Intel Turbo Boost Technology frequency varies depending on workload, hardware, software and overall system configuration.

Due to varying power characteristics, some parts with Intel Turbo Boost Technology 2.0 may not achieve maximum turbo frequencies when running heavy workloads and using multiple cores concurrently.

Availability and frequency upside of Intel Turbo Boost Technology 2.0 state depends upon a number of factors including, but not limited to the following:
•Type of workload
•Number of active cores
•Estimated current consumption
•Estimated power consumption
•Processor temperature

When the processor is operating below these limits and the user's workload demands additional performance, the processor frequency will dynamically increase until the upper limit of frequency is reached. Intel Turbo Boost Technology 2.0 has multiple algorithms operating in parallel to manage current, power, and temperature to maximize frequency and energy efficiency. Note: Intel Turbo Boost Technology 2.0 allows the processor to operate at a power level that is higher than its TDP configuration and data sheet specified power for short durations to maximize performance."