Why do whole house filter setups need 4 shutoff valves?

You realy only need three ball valves for a proper setup, not four. That fourth valve in diagrams is almost always either pure redundancy from an over-cautious DIYer, or it's a flush valve specifically meant for blowing down a spin-down sediment trap without taking the canister off.

To make a functional bypass, three is the magic number: one before the filter bank, one after the filter bank, and one normally closed in the middle of the bypass loop. If you only put a single valve on the bypass and kill the whole-house main to swap filters, you run into two headaches fast. First, nobody in the house gets running water while you're working, wich gets annoying if you run into a stuck canister or need to run to the store for a pinched O-ring. Second, and more importantly, water will backfeed right out of your house lines. Every pipe upstairs holds gallons of water, and the second you spin that filter housing off, head pressure will push all that water backward down into your basement and all over your feet. That downstream valve right after the last filter is what locks the house's water column in place so you aren't fighting a siphon. Close the inlet, close the outlet, open the bypass, and the rest of the house still has water while you relieve pressure with the red bleed button and drop the sumps in peace. Save your copper and your solder. Stick with three **full-port ball valves** for the loop itself. If you've got a reusable pre-filter with a hose barb drain at the bottom, that drain valve counts as a valve, but putting an extra isolation valve between individual filter stages is just burning money on fittings.

Have you ruled out how often you'll actualy touch that first stage compared to the rest? If your pre-filter catches heavy silt or rust, you might find yourself servicing it every few weeks while the rest of the bank sits untouched for six months. Having a valve right after the sediment stage lets you service just that housing without depressurizing the whole rack and burping air thru your entire plumbing system every time. Sure, it's an extra fitting and two joints, but have you checked your water quality enought to know you won't get annoyed bleeding all three sumps monthly?

Look closely at wheather that fourth valve in the drawing is acutally inline or a tee'd boiler drain. If you're running a Big Blue rack with a carbon block—say a Pentair RFC-20BB or KX Matrikx CTO—you never want to send water into your domestic lines immediately after a swap. You have to flush out gallons of black carbon fines first. Having a 3/4" Apollo ball valve with a hose bibb right before that downstream isolation valve lets you purge straight into a 5-gallon bucket before opening flow back up to your water heater and fixtures. That’s usally what people are drawing.

If you're worried about burning thru copper fittings and solder, take a look at the Webstone Pro-Pal bypass valve. It basially packs that entire three-valve bypass loop into a single forged brass casting.

I used the 1" Webstone 50614W on my basement filter setup. It eliminates about six separate solder joints and saves you from building an ugly spiderweb of tees and 90s. Way cleaner install, fewer potential leak points, and you still get full isolation.

If that fourth valve is situated in-line rather than acting as an auxiliary drain port, it is frequently placed there to mitigate severe hydraulic shock upon system recommissioning. Snapping a standard full-port ball valve wide open against empty twenty-inch filter sumps creates an instantaneous transient pressure wave that can readily deform pleated sub-micron media, channel virgin granular activated carbon, and blast high-velocity air pockets straight into downstream fixtures. Having an extra intermediate valve permits a deliberate, throttled repressurization of the housings before subjecting the entire assembly to full domestic static line pressure.

Quick update — checking out that Webstone Pro-Pal valve was a lifesaver, definitly using that to cut down on all the copper soldering. But I'm still stuck on how to handle the boiler drain for flushing carbon fines. shoud that tee go right off the final filter outlet before the shutoff, or is there a better spot to tie it in?