A multi-user visible-light communication (VLC) system based on 4  ×  4 multiple-input multiple-output (MIMO) was investigated. The system consists of four light-emitting diodes (as a transmitter) and four photodetectors (as a receiver). The proposed system was simulated for two, three, and four users. The objective of this study is to optimize the system power consumption and to utilize the sum rate on the system coverage area. By expanding the system from a 2  ×  2 MIMO-VLC to a 4  ×  4 MIMO-VLC, the results demonstrate that the total sum rate of the system can be improved. The findings reveal that the 4  ×  4 MIMO-VLC achieves a maximum sum-rate enhancement of 143% compared with the 2  ×  2 MIMO-VLC. The results also show that the efficiency of 4  ×  4 MIMO-VLC system with non-orthogonal multiple access (NOMA) and gain ratio power allocation (GRPA) was increased by around 181% compared to 2  ×  2 MIMO-NOMA-based VLC with GRPA in combination. For the 4  ×  4 MIMO-NOMA-VLC device with GRPA, the sum bit rate for two, three, and four users was increased by 40%, 33%, and 24%, respectively. Through this study, it can be concluded that GRPA scheme plays an important role in utilizing MIMO-VLC system sum rate of the MIMO-VLC system.