In this Letter, a ring-core erbium-doped fiber (RC-EDF), with a two-layer erbium-doped structure, supporting up to the fourth-order orbital angular momentum (OAM) mode was designed and fabricated for the OAM mode multiplexed amplification. Using the RC-EDF, the amplification of the third- and fourth-order OAM modes with an ultra-low differential mode gain (DMG) has been demonstrated by observing both the modal intensity and phase distribution and measuring the modal gain under the fundamental mode core pumping. The measured average gain of four modes' (OAM±3,1 and OAM±4,1) multiplexed amplification is >19dB, covering the C band, and the DMG is <1dB. Additionally, the gains of two conjugate OAM modes are almost the same under different pump power, regardless of whether they are amplified simultaneously or separately.