4 week old kitten with dark orange/yellow diarrhea?

My cat had a litter of 5 four weeks ago. All of them had been seen by a veterinarian and were supposedly healthy. However, one of them just had a bout of yellow/dark orange diarrhea from out of nowhere.

Their mother is an indoor-only cat. Their environment is kept as clean as possible. What could have caused this, and will it be safe to wait a bit and see how things go before taking him to the vet? Any help/advice would be appreciated!

Many things could have caused it, but you need to take him to a vet to find out and properly treat it. Kittens that age cat decline very fast so don't risk it and take him in.

You can perhaps begin with phoning this your vet?

You are his paying patient, so there is nothing peculiar with you phoning.  And yes, if the vet wants to see the kitten, or even all of them, drive in.

Begin also preparations for making home made pedialyte and such, if.

The easiest way to make a home pedialyte is take water after cooked rice  (you have rice and much water, cook well over the usual time, and take away the water for using. The rice you can eat yourself even if it is a little too well done).

Add some kitchen salt,

1 litre + 1 tea spoon (3g) kitchen salt.

a classic pedialyte is

1 litre water

1,5 soup spoon (15g) dextrose (glucose sugar) - other sugars like honer also OK, but dextrose best.

1 tea spoon (3g) salt, preferably "minerale salt", but common kitchen salt oK.

Boil up with stirring, cool down.  Use for kittens and kids in cases of severe diarrhea, throwing up or similiar...

Last but not least.  If a kitten seems to be weak or even fading, you can try to boost some energy by giving some wetted dextrose/honey/white caro syrup on the gum.

The fading is often caused by low blood sugar contents, so this helps directly, and with dextrose - immediately.

Good luck!

Thank you both for your suggestions. I did call the vet as soon as I noticed it, but their latest appointment was for tomorrow, so I'm going then. Concerning the diarrhea, the receptionist told me it's urgent for a kitten so young to be seen, as they can lose fluids very easily. However, he's nursing/playing as normal so fingers crossed he'll be OK for tomorrow's appointment!