When you say 4 stage or 6 stage what do you mean? They come in different configurations these days and you could easily get more stages at one side of the membrane or the other.
Now bay area water is relatively clean at the water treatment plant, any sort of "dirtiness" (turbidity) will be in the pipes between there and your faucet, but overall we're typically better off than those who use well water. That said the biggest issue with our water is chloramines and that has to be dealt with before your RO membrane, as such you want to make sure you have proper filters that can deal with it. Standard carbon filters, the cheap ones you can get, can deal with chloramines but they can't process much of it, now at the other end of the spectrum are your "Chlorine Guzzler" type filters and they can process lots of chloramines however they also do push the scale of expensiveness really high, and there are filters in between like the BRS Universal carbon block which processes a fair amount of chloramines but aren't nearly as expensive as the high end. IMO, carbon is the backbone of your RO unit, yes the RO unit gets out "dirty" stuff that gets through the filters, yes DI resin absolutely cleans up everything that's left but the carbon blocks stop the most deadliest thing from getting into your tank which is chloramine, the other stuff can possibly make algae grow and your tank look icky, but chloramine kills life.
That all said tl;dr version of the minimum setup I would do
-Prefilters - 1 sediment, 2 carbon blocks, if you open the wallet up for the high end chloramine block you don't need 2 of those, but should have 1 other block
-RO membrane - Shoot for 75gpd, no reason to go smaller, and going to a larger membrane allows more stuff to pass it depleting the next stage quicker.
-DI resin - mixed bed, this polishes off the last bits of what gets through the membrane and leaves you with clean pure water.
-Clear canisters - I used solid white canisters for most of my reefing life, and I largely was guessing at how dirty the filters were sometimes when I change them I wish I water longer, other times I wonder why I put a brown filter into it
, having visual cues can help you not get lazy with changing filters.
There are all sorts of extras you can add, dual membranes, a second DI chamber, separate DI in 3 stages, multiple sediment filters, booster pumps, inline TDS meters. All of it depends upon what you want to do, and luckily all of it can be added on to basic units. Overall though I don't think there is much difference between actual UNITS that are sold, some have different bells and whistles, maybe they give you better initial filters (you need to look into that), make sure it ticks off the minimum of what YOU want first and realize it may seem expensive to make what's relatively cheap out of the faucet, but the cleaner you have that initial water, the more less problems you're going to have in your tank.
