Does Broken Keel have a 4-piece bonus?

No. Broken Keel is a Planar Ornament set: it has two pieces, a Planar Sphere and a Link Rope, and the 2-piece bonus is the whole set.

Planar SphereInsumousu's Whalefall Ship

Broken Keel

Planar Ornament set · Honkai: Star Rail

2★ · 3★ · 4★ · 5★

One tier

2-piece

Increases the wearer's Effect RES by 10%. When the wearer's Effect RES is at 30% or higher, all allies' CRIT DMG increases by 10%.

A Planar Ornament set is two pieces — the Planar Sphere and the Link Rope — so the 2-piece bonus is the whole set. There is no 4-piece tier to reach.

2 slots

Planar Sphere
Insumousu's Whalefall Ship
Encapsulated in the plane is the Insumousu Autonomous Region of Thalassa. The Xianzhou Daiyu had crashed here, inspiring the civilization of Insumousu.
Link Rope
Insumousu's Frayed Hawser
The fallen Xianzhou Daiyu is forever anchored in a foreign land. Perhaps it is time to untie its mooring ropes and let it return home.

What is the Broken Keel 2-piece bonus?: Increases the wearer's Effect RES by 10%. When the wearer's Effect RES is at 30% or higher, all allies' CRIT DMG increases by 10%.
Does Broken Keel have a 4-piece bonus?: No. Broken Keel is a Planar Ornament set: it has two pieces, a Planar Sphere and a Link Rope, and the 2-piece bonus is the whole set.