can Bmpcc4k be used without Internal IR-Cut?

I was wondering if we could use a Blackmagic Pocket cinema camera with internal IR_CUT removed with screw on IR-filter only?

As far as I know, the infrared filter is part of the sensor glass protecting your sensor surface. It’s a mild attenuation of infrared wavelengths and not a Cut filter. You likely should not remove this glass as it can affect how optics focus such as the Metabones SpeedBooster designed for various BMD cameras.

Most modern neutral density filters are now IRND meaning they attenuate visible and infrared wavelengths to the same degree. But you may still use an additional true infrared cut filter where there is heavy infrared radiation and materials or objects (such as rayon, nylon, leaves of trees and bushes) that radiate heavy infrared.

I was wondering if we could use a Blackmagic Pocket cinema camera with internal IR_CUT removed with screw on IR-filter only?

It's possible.

The sensor cover glass made by BMD is a mild IR filter, and is designed to protect the sensor itself from dirt and other contact damage.

If you remove it, there's a mild change to the IR response of the camera, and you may find your lenses may not collimate or track focus accurately. Adding or subtracting from behind the lens in this way can greatly affect lenses being able to focus accurately according to wither witness marks on the lens itself, or be able to even hit infinity on some wider focal lengths. It's a bit of luck in this regard as to it being a problem. If you know what you're doing you can also counteract this as well, though any re-shimming of your lenses will now mean they won't work so well on another camera that DOES have it's IR filter in place.

JB

If you remove the ir filter, replace it with clear glass. This will correct the backfocus and protect the sensor from contamination.

Good Luck

If you are aiming at IR shooting, don't expect as much of an effect as the Foveons have (for stills).