Why is there a yellowish tint on my display of my new iPad Pro M4 13in?

Turn this off:

True Tone,* which is on by default, uses advanced sensors to adjust the color and intensity of your display to match the ambient light, so that images appear more natural.

Adjust the brightness and color temperature on your iPhone or iPad - Apple Support

to me, all True Tone does is make the screen yellow.

You are asking for advice and help on a user to user forum. Suggestions will be provided if you don’t explicitly state what you have done to try and improve the issue before posting your question. We can’t assume you have done anything (surprisingly most people that post on here asking for help have not done even what some may consider to be basic steps…). Even if you don’t like the fair suggestions provided by someone there is no reason to respond in a rude manner. You can simply state that you already tried that and it did not help. Changing the device in the store under the exchange/return period does not automatically mean you or the store tried turning off night shift and True Tone.

Ultimately, your option is to go back to the store and try another exchange (and actually check the device before leaving the store to save yourself another trip or disappointment after going back home), return the device for a refund if you fear that all exchanges will have the same issue, or keep it and work with Apple support to determine if the issue is one that may be addressed with a future ipadOs update if possible or a future repair of Apple determines there is a faulty part. Which one is the best choice is subjective and something only you can decide which would work best for you.

All iPad Pro displays are individually calibrated for brightness and white-point during manufacture of the iPad.

Given your description - and experience in receiving two iPad's that exhibit consistent behaviour - a logical explanation might be that the in-store iPad with which you have made comparison may have different settings and/or might be running a different version of iPadOS.

Given that in-store demo models will be running a specific demo-version of iPadOS, some differences in what you see might be reasonably anticipated.

Be aware that you can adjust the white-point in iPad settings:

Settings > Display & Brightness > Night Shift > Colour Temperature - more the slider towards Less Warm

Many users comment the default slider mid-position appears too warm (shifting the screen to the yellow part of the visible spectrum) in some lighting conditions. Moving the slider to the left shifts the display to the bluer (i.e., brighter white) spectrum.

It is perhaps likely that the display settings of the in-store demo iPad was configured in this manner.

Where in your question did you post that?

If you don’t put it in your post, we don’t know it.

Since you want to be rude when someone tries to help you, good luck