Hi, felt I had to chime in to clarify mistaken information regarding MT4 volume.
MT4 volume is not comprehensive (there is a difference) where is davem1979 is correct about volume information being localized to the Forex broker and that Forex broker's Tier 2 liquidity provider (the bank or group of banks that Forex broker's ECN has agreements with).
As a heavy user of MT4 myself (around 7 years), I've found that any volume information from MT4 is tick volume, not price or open interest volume. Using volume from MT4 is not comprehensive of the entire Forex mark because volume (and price) information is decentralized amongst the Tier 1 Interbanks and the Tier 2 Bank that your MT4 broker has agreements with. There is no equivalent of a complete "Level 2" for Forex markets because its decentralized.
While there have been religious debates between the merits of "tick" volume and "order/price/open interest" volume (call it what you will), that's for another discussion. It is possible to code indicators looking at MT4's tick volume, but its accuracy depends on how big your broker's order book is and if they've got enough users/volatility/critical mass to accurately reflect the Forex market in order for this information to be of use to you.