I'm replacing the piping that goes from my kitchen sink. From galvanized and cast to pvc. The old line went into the basement then 45 down into the top of some 4" cast fittings. I guess what I'm wondering is do I have to replace the 4" pipe if the only things on this line are the sink and a washing machine? Can I just use a donut to go into the 4" cast hub at the basement floor?

If only the kitchen sink and washing machine are on this line, 2" is big enough by code. And yes, a donut into the cast iron hub will work just fine for transition to pvc.

If there is a lack of venting on the washer or sink, it wouldn't hurt to run three inch, but four would definatley be overkill.

My area went from 2" to 3" on kitchen drain lines especially if there's a disposal involved. If a washer's on the same liner I'd recommend using 3" PVC. Yes! You can use a 4 X 3" donut connect to the 4" cast iron hub.
Dad asks a pertinent question. Are the sink and washer vented?
Back to you, Tom

No, they are not vented. There is a disposal also.

In that case I would install a AAV ( Air Admittance Valve) (see image) after each trap. Good luck, Tom

Even the washing machine? One more question, I already bought 4" fittings and pipe because "that's what was there before". Should I use them because I've already got them, or exchange them for 3"? What would you guys do? Thanks for all your help guys.

Yes, even the washer. If you already have the pipe and fittings, might as well use them. Only Advantage of smaller pipe would be space consumption and materials would be a bit cheaper for smaller pipe and fittings. Four inch pipe would actually have one advantage, as it would allow for circulation of air, acting as somewhat of a vent, but I would still follow Toms advice and install aavs on sink and washer.