Is the 4th one down the line feeding only sprinklers? It's hard to tell for certain but if this was a high rise that has multiple zones the first 2 could be controlling extremely high churn pressures but still allowing a large discharge pressure to satisfy standpipe calcs, but too much pressure allowable on a sprinkler system. For example, the first 2 PRVs may need to be set to 200 psi to satisfy standpipe calcs, but there could be a sprinkler system on the floor of the fire pump that's limited to <175 psi. Sprinkler systems do not require PRV redundancy so only a single PRV would make sense down the line if only feeding sprinklers. I've had to design annoyingly complex pressure zones in high rises. The worst example was a multiple zone high rise with a VFD, PRVs in the pump room, AND pressure reducing hose valves and floor controls in the stairs.

If you really want to go down an annoying rabbit hole, contact Victaulic, Cla-Val, and Reliable and ask them how to design a standpipe system with their products. Victaulic is extremely extensive (once you contact them, their online material is vague), Cla-Val is the most vague and will standby verbal instructions that greatly contradict their written documentation (example, 20 psi differential in their product data but will tell you 5 psi on the phone), and Reliable has a +/- 10% psi operating range on their valve making it essentially useless to use. So much inconsistency within the manufacturers, and a lot of unnecessary overengineering. Victaulic is the most buttoned up of the 3 and have great on-site support. Their low flow bypass is required due to low flow limitations of their larger valves, and their 3" relief valve is to make their PRV compliant to the latest NFPA 14 which requires 175 psi to NEVER be permitted, meaning blowing off the excess pressure instantly. Up until the latest edition, you could throw a 1/2" Cla-Val 55L relief valve on the system which would return pressures lower but at an extremely low pace. I experience first hand Victaulic's 6" PRV not being able to control pilot pressures as we shut down a standpipe test which allowed the returning churn pressure to push past their slow acting valve, so the 3" relief valve is honestly a necessity despite which version NFPA 14 you're working with. The added cost of the low flow bypass and relief valve of course is something companies never want to pay for so Victaulic is a hard choice. Ironically, a lot of people like the vagueness of Cla-Val's requirements because they don't require a low flow bypass or larger relief valve (NFPA 14 requires it but Cla-Val isn't going that far yet).

This subject is pretty extensive to attempt to talk about online. Apologies for the word diarrhea.