This document provides a formal definition of the modelling language HLL - High Level Language. HLL is a declarative stream-based language supporting a large variety of types and operators. It is suitable for modelling discrete-time sequential behaviors and expressing temporal properties of these behaviors. In 2018 the HLL Forum initiative started as a working group of tool providers and users of HLL. New features of HLL (compared to version 2.7) were developed jointly by the HLL Forum. This document is a prerelease (proposal) by Prover to HLL Forum of an upcoming version 4.0.