Hi M.,
First, I completely applaud your efforts and desire to consider cloth diapers! I used them for both of my boys (now 16 and 12) and never regretted it. (I always had disposable on hand for traveling or, sometimes, for overnight. They are convenient at times!)
How many depends largely on how often you want to have to do a load of diapers--you'll probably want to do them by themselves. I honestly don't remember how many I had, but it seems like 40-60 at least. That way, the diaper pail could comfortably fill up--after being washed out in the toilet first--and still have plenty of clean folded diapers stacked up ready to go. Clean diapers are so handy for burp cloths and other emergencies!
I liked the night pads for when I went grocery shopping or out some place...and for night time, of course...better than folding an extra diaper between the regular one and the diaper cover because it seemed to fit better and be more comfortable. Maybe a dozen of those or so.
While you don't need nearly as many diaper covers as diapers, it is still nice to have some clean ones handy if needed. Again, I don't remember specifically, but perhaps a dozen of those as well. I DO remember liking the velcro tabs and being able to secure the diaper with the covers instead of the diaper pins we used to have to do!
The nice thing is that you can always buy more diapers or covers if it suits you. After a little while, you'll integrate the cloth lifestyle seamlessly into your routine and probably be delighted with the results. I never had problems with diaper rash using the cloth, but my sons were sometimes reactive (mildly) to disposable. However, I was VERY careful to only use a detergent without any fillers or potential allergens. The only one I trusted at that time was SA-8 from Amway/Quixtar. Now there are probably other excellent choices, but I tend to stick to what works the best for us.
Remember, you'll probably want to change cloth more often than typically is done with disposables. So...a little more work, but I found my sons stayed very happy and healthy that way. Also as a bonus: they were both a snap to potty train, unlike their peers whose mothers were often quite distressed. Of course, since this is your 4th, you no doubt already have that one down!
I guess the main advice I have is to appreciate the positives the cloth diapers provide when it is less convenient than tossing a disposable into the trash (like when you are washing out a dirty diaper in the toilet, or facing a load of clean diapers out of the dryer that have to be folded). Then, when it really makes more sense to use disposable, go ahead with a sense of relief and appreciation for the choice. You know in your heart and mind that you are doing a significant part to protect our planet for your precious youngsters!
Be blessed!
C.