We present the results of a new study of dust attenuation at redshifts 3<z <4 based on a sample of 236 star-forming galaxies from theVANDELS spectroscopic survey. Motivated by results from the FirstBillion Years (FiBY) simulation project, we argue that the intrinsicspectral energy distributions (SEDs) of star-forming galaxies at theseredshifts have a self-similar shape across the mass range 8.2 ≤log(M⋆/M⊙) ≤ 10.6 probed by oursample. Using FiBY data, we construct a set of intrinsic SED templateswhich incorporate both detailed star formation and chemical abundancehistories, and a variety of stellar population synthesis (SPS) modelassumptions. With this set of intrinsic SEDs, we present a novelapproach for directly recovering the shape and normalization of the dustattenuation curve. We find, across all of the intrinsic templatesconsidered, that the average attenuation curve for star-forming galaxiesat z ≃ 3.5 is similar in shape to the commonly-adopted Calzettistarburst law, with an average total-to-selective attenuation ratio ofRV = 4.18 ± 0.29. In contrast, we find that an averageattenuation curve as steep as the SMC extinction law is stronglydisfavoured. We show that the optical attenuation (AV) versusstellar mass (M⋆) relation predicted using our method isconsistent with recent ALMA observations of galaxies at 2 <z <3in the HubbleUltraDeepField (HUDF), as well as empirical AV -M⋆ relations predicted by a Calzetti-like law. In fact,our results, combined with other literature data, suggest that theAV - M⋆ relation does not evolve over theredshift range 0 <z <5, at least for galaxies withlog(M⋆/M⊙) ≳ 9.5. Finally, wepresent tentative evidence which suggests that the attenuation curve maybecome steeper at lower masses log(M⋆/M⊙)≲ 9.0.