Couldn't find exact CMYK for blue RGB colour

Without an associated ICC profile, the numbers in an image file are near meaningless. 

IF you are converting from RGB to CMYK [as you seem to be] you are using 2 x ICC profiles, an RGB one for the original document and a CMYK one for the destination in the conversion.

If you don't set those - defaults are used.

If you just go to image/ mode CMYK then the conversion is between the file's colourspace (RGB ICC) and the default CMYK as set in "color settings".

The correct choice of CMYK destination profile is totally dependent on the print method used for the final result. 

OK so that’s one issue

There other is that some RGB colours cannot be printed because ink on paper is not capable enough (the CMYK ICC profile defines the colour range available).

Photoshop's "Gamut Warning" helps predict such issues.

"HEX" is just another way of expressing an RGB colour 

Let's do a test:

Open Photoshop, make a new document and set the colour to HEX 4B7EEF (which is the same thing as RGB 75,126,239)

Now go to Photoshop's Edit "assign profile" dialog and try assigning different RGB profiles, eg Adobe RGB or sRGB and watch the appearance change.

You ARE doing this on a calibrated profiled display I hope? Otherwise onscreen assessment is somewhat futile.

As you can see, the assigned RGB ICC profile / colourspace alters the appearance, this proves, I hope, that RGB and HEX numbers are meaningless without an associated ICC profile.

So your colour may be HEX: 4B7EEF - BUT in what colurspace? sRGB? Adobe RGB? something else? You need to know.

Next we'll try a CMYK softproof to predict the conversion

Go to Photoshop's View/proof setup/custom

Select Relative Colorimetric and try "coated FOGRA 27 - - " see how the colour fades when the conversion is predicted, that's because of gamut, the gamut of tthe original is outside the gamut of the CMYK colour space.

If you activate View / Gamut warning with sofr proofing active it will show you the out of gamut colour. On my system your HEX: 4B7EEF is entirely out of the gamut of coated FOGRA 27 - - which is a pretty large colourspace as CMYK colourspaces go.

I hope this helps you understand the issues
neil barstow, colourmanagement net :: adobe forum volunteer
google me "neil barstow colourmanagement" for lots of free articles on colour management
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RGB numbers are close to irrelevant without a Color Space definition (and the same goes for CMYK numbers). 

So what is the RGB Color Space – sRGB, AdobeRGB, eciRGB v2, …? 

And what is the CMYK target space? 

Quite possibly the color in question is out of gamut for the target space.