Sub Zero Covers
Tan insulated backflow cover in a retail parking-lot island, fitted around the assembly with the yellow fire department connection riser left clear.
Retail center — FDC riser left clear of the cover

By assembly and size

4" RP backflow cover

4" RP backflow cover sizing starts with the assembly on the tag. At 4", 10 RP assemblies from the brands we pattern for are common: Watts 909 / LF909, Watts 957 / 957Z, Febco LF860 Large, Zurn Wilkins 375 / 375A / 375AST, Zurn Wilkins 475 / 475V, Ames 4000SS, Ames Colt C400, Apollo / Conbraco RPLF4A large, Deringer (Ames) 40, and Deringer (Ames) 40G / 50G. The price list maps this pipe size to one stock cover, and the published dimensions below say which one to pull.

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What "4" RP" covers on a counter

Reduced pressure zone (RP / RPZ). At 4" the assemblies your customers bring in are Watts 909 / LF909, Watts 957 / 957Z, Febco LF860 Large, Zurn Wilkins 375 / 375A / 375AST, Zurn Wilkins 475 / 475V, Ames 4000SS, Ames Colt C400, Apollo / Conbraco RPLF4A large, Deringer (Ames) 40, and Deringer (Ames) 40G / 50G. They are not the same size: the Zurn Wilkins 475 / 475V is 18.3125" end to end and the Watts 909 / LF909 is 55.3125", so "a 4" RP" is not a single envelope.

The stock cover for 4"

The April 2026 price list maps 4" to 85 × 40 (DS8540), with the N-shape NS4 for N-pattern installs. That cover was built around this size class; the table below shows how each assembly sits inside it.

Relief valve drainage

Every assembly on this page has a relief valve that discharges downward from under the body. Leave that path open. Close the cover to the pad around the assembly, never under the relief port.

When to measure instead

Unions, strainers, OS&Y gates, and wall-tight mounts all move an assembly off its spec sheet. If the tag is gone or the install is odd, measure it or send photos for a custom cover. For the type as a whole see RP covers; for another size, start at the fitment hub.

Sub Zero Covers sells through distributors only. Distributor accounts see pricing and availability in the portal; everyone else orders through their supply house, or applies for distributor access.

Assemblies at this size

4" reduced pressure zone assemblies and the cover that fits

Dimensions in inches from each manufacturer's spec sheet with standard shutoffs. Length end to end; heights from the pipe centerline. The cover column is the closest stock fit by published length; confirm against the assembly in front of you.

AssemblyLengthAbove CLBelow CLClosest stock cover
Watts 909 / LF90955.3125"22.75"6"85 × 40 (DS8540) · N-shape NS4
Watts 957 / 957Z33.75"22.75"7"85 × 40 (DS8540) · N-shape NS4
Febco LF860 Large46.25"12.5"11.5"85 × 40 (DS8540) · N-shape NS4
Zurn Wilkins 375 / 375A / 375AST37.625"14.5"11"85 × 40 (DS8540) · N-shape NS4
Zurn Wilkins 475 / 475V · N-pattern18.3125"——N-shape NS4
Ames 4000SS40"22.75"10.5"85 × 40 (DS8540) · N-shape NS4
Ames Colt C40033.75"22.75"7"85 × 40 (DS8540) · N-shape NS4
Apollo / Conbraco RPLF4A large37.6"22.8"9.8"85 × 40 (DS8540) · N-shape NS4
Deringer (Ames) 4018.7"7.9"8.7"85 × 40 (DS8540) · N-shape NS4
Deringer (Ames) 40G / 50G33.9"22.5"9.3"85 × 40 (DS8540) · N-shape NS4

Common questions

Which cover fits a 4" RP?
The 85 × 40 (DS8540) is the price-list match for 4". N-pattern installs take the N-shape NS4 instead.
Do all 4" RP assemblies take the same cover?
Not always. At this size the published lengths run from 18.3125" (Zurn Wilkins 475 / 475V) to 55.3125" (Watts 909 / LF909). Order by model.
Is this a heated enclosure?
No. Sub Zero Covers are insulated, unheated covers (R-13 standard, R-30 extreme weather). They are not ASSE 1060 listed enclosures. Where a spec calls for a Class I heated enclosure, ask sales before quoting.

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