We report muon spin relaxation (SR) measurements using single crystals of oxygen-intercalated stage-4 La 2 CuO 4.11 ͑LCO:4.11͒ and La 1.88 Sr 0.12 CuO 4 ͑LSCO:0.12͒, in which neutron scattering studies have found incommensurate magnetic Bragg reflections. In both systems, zero-field SR measurements show muon spin precession below the Néel temperature T N with frequency 3.6 MHz at T→0, having a Bessel function line shape, characteristic of spin-density-wave systems. The amplitude of the oscillating and relaxing signals of these systems is less than half the value expected for systems with static magnetic order in 100% of the volume. Our results are consistent with a simulation of local fields for a heuristic model with ͑a͒ incommensurate spin amplitude modulation with the maximum ordered Cu moment size of ϳ0.36 B , ͑b͒ static Cu moments on the CuO 2 planes forming ''islands'' having typical radius 15-30 Å, comparable to the in-plane superconducting coherence length, and ͑c͒ the measured volume fraction of magnetic muon sites V increasing progressively with decreasing temperature below T N towards V ϳ40% for LCO:4.11 and 18% for LSCO:0.12 at T→0. These results may be compared with correlation lengths in excess of 600 Å and a long range ordered moment of 0.15Ϯ0.05 B measured with neutron scattering techniques. In this paper we discuss a model that reconciles these apparently contradictory results. In transverse magnetic field SR measurements, sensitive to the in-plane magnetic field penetration depth ab , the results for LCO:4.11 and LSCO:0.12 follow correlations found for underdoped, overdoped and Zn-doped high-T c cuprate systems in a plot of T c versus the superconducting relaxation rate (T→0). This indicates that the volume-integrated value of n s /m* ͑superconducting carrier density / effective mass͒ is a determining factor for T c , not only in high-T c cuprate systems without static magnetism, but also in the present systems where superconductivity coexists with static spin-densitywave spin order.