In this paper, a new proposed space-time code for four-time slots and two transmitters for uplink transmission in a mobile system is presented. The mobile system needs three or four-time slots for data transmission. Also, this code is applied for multi-input multi-output (MIMO) fading channel. The proposed orthogonal space-time code achieves the properties of space-time code such as a full rate, full diversity, and non-vanishing minimum determinant value (MDV). The coding gain of this code is high because it achieves the trace or determinant criterion. Also, the proposed code needs a linear maximum likelihood (ML) decoding to decode four real symbols. Simulation results show that the performance of the proposed code is better than the performance of the previously presented schemes that are used in a mobile system for different modulation and diversity.