Depends if you are a broker or a trader,
If you are a broker, you can buy a server license of MT4 for a pretty penny and connect large liquidity providers with traders and thereby profit by pocketing the bid/ask spread (and/or for Non-dealing desks by charging commissions). If you are a trader, you can download the client side of MT4 for free, sign up with a broker and use the software to either:
(a) Manually place trades as you would with any other trading platform.
(b) Design and run automated or mechanical currency trading systems, popularly known as robots by using the software’s own scripting language called MQL4.
(c) Use some type of combination of the above two.