Quaternary In-0.52(AlxGa1-x)(0.48)As(0 less than or equal to x less than or equal to 1) compounds lattice-matched InP substrates were grown and characterized. The engery bandgap (E(g)) of these In-0.52(AlxGa1-x)(0.48)As compound were (0.806+0.711x) eV. The conduction band discontinuity (Delta E(c)) at the In-0.52(AlxGa1-x)(0.48)As/In0.53Ga0.47As (x=0.9 and 0.75) heterojunctions was approximately (0.68 +/- 0.01)Delta E(g). The quaternary In-0.52(Al0.9Ga0.1)(0.48)As/In0.53Ga0.47As Q-HEMTs with a gate-length of 0.8 mu m revealed an extrinsic transconductance (g(m)) of 295 mS/mm, an f(T) of 35 GHz and an f(max) of 76 GHz. This Quaternary HEMT was proven to be more reliable, comparing with the conventional InAlAs/InGaAs HEMTs.