Weakly-coupled mode division multiplexing (MDM) technique is considered a promising candidate to enhance the capacity of an optical transmission system, in which mode multiplexers/demultiplexers (MMUX/MDEMUX) with low insertion loss and modal crosstalk are the key components. In this paper, a low-modal-crosstalk 4-mode MMUX/MDEMUX for the weakly-coupled triple-ring-core few-mode fiber (TRC-FMF) is designed and fabricated with side-polishing processing. The measurement results show that a pair of MMUX/MDEMUX and 25 km weakly-coupled TRC-FMF MDM link achieve low modal crosstalk of lower than -17.5 dB and insertion loss of lower than 11.56 dB for all the four modes. Based on the TRC-FMF and all-fiber MMUX/MDEMUX, an experiment for 25 km real-time 4-mode 3-λ wavelength division multiplexing (WDM)-MDM transmission is conducted using commercial 400G optical transport network (OTN) transceivers. The experimental results prove weakly-coupled MDM techniques facilitate a smooth upgrade of the optical transmission system.