I would definitely use a white underbase. On dark shirts, I base just about everything except black ink. Trap the base by adding a .5pt white outline to everything on your underbase screen design. That's a super tiny bit of trap, smaller than a human hair, but that's all you really need for easier registration, and no white peeking out around the edges.
It's usually not desirable to have 4 layers of ink. That will make a very thick, rigid and heavy print. It's not very pleasant to wear. Most people think that a lighter, softer print is more desirable. So try to avoid doing that.
Yellow is tricky, since it tends to be very translucent. I pretty much always use Wilflex Golden Yellow ink, whenever I need a gold or a yellow. It's more opaque and always looks good. No matter what the customer requests, I just send my proof to the customer, colored and listed as 123 and they pretty much never notice it or dispute it.