Stoichiometric Ni2MnGa and sligthly off-stoichiometric Ni-Mn-Ga Heusler systems are prototype magnetic shape memory materials with parent cubic L21 structure and extraordinary magnetomechanical properties in lower-symmetry martensitic phases. Upon cooling from cubic parent, a wide range of transformations occur with typical sequence cubic L21->10M->14M->NM, where 10M and 14M denote modulated martensite phases and NM non-modulated tetragonal martensite phase. Ab initio calculations suggest the true ground state to be 4O, i.e. orthorhombic structure with modulation period of 4O atomic planes. We will study the transformation sequence in order to determine structural relation between transforming phases in whole crystal, the nucleation of the phases and their coexistence and changes with magnetic structure as well as the atomic arrangement. We aim to resolve the dilemma whether the intermartensite transitions are true phase transformation or detwinning process and additionally we will search for the newly predicted 4O ground state at low temperatures.