How to always keep the turbo clock?

I agree with Xaltar, you are limited to the maximum four core Turbo speed, which for your CPU is 3.7GHz.

Some ASRock boards have an option in the UEFI, OC Tweaker section, called CPU OC Fixed Mode. I don't see it listed in your board's manual, but the manuals tend not to be kept up to date with options that were added in a UEFI update. Which means there is a possibility it is in your board's UEFI. It may not be available on Intel 8 series chipset boards like yours, or if you aren't using a 'K' CPU.

This option would do what you wanted to do, keep all the core multipliers at the maximum multiplier set for all the cores.

But I've had odd results with this option, when using a stock maximum multiplier. At idle and low loads on the CPU, the multipliers for all cores would stay at the highest value constantly. But run a stress test and the core speeds would drop to the standard max for all cores running a maximum. At least that is what I saw on a Z97 board and Haswell 'K' processor.

There is no option to set constant turbo. Turbo does not set all cores to the turbo frequency but rather adjusts depending on the workload, something like this: