This paper presents a dedicated search for exotic decays of the Higgs boson to a pair of new spin-zero particles, H→aa , where the particle a decays to b -quarks and has a mass in the range of 20–60 GeV. The search is performed in events where the Higgs boson is produced in association with a W boson, giving rise to a signature of a lepton (electron or muon), missing transverse momentum, and multiple jets from b -quark decays. The analysis is based on the full dataset of pp collisions at s=13TeV recorded in 2015 by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 3.2 fb-1 . No significant excess of events above the Standard Model prediction is observed, and a 95% confidence-level upper limit is derived for the product of the production cross section for pp→WH times the branching ratio for the decay H→aa→4b . The upper limit ranges from 6.2 pb for an a -boson mass ma=20GeV to 1.5 pb for ma=60GeV .