My MT4 chart prices are different than the Market Watch price too. I noticed this earlier tonight when I clicked on the Tick Charts tab on the Market Watch window. So far I've noticed a price difference of 1 to 1.1 pips.
The price in the Market Watch Tick Chart tab is the same as my broker's price. I checked my broker's price via web browser. Also, documentation on my broker's site explains that my broker's price is bid plus ask divided by two. And this is the price I see in the MT4 Market Watch window on the Tick Chart tab.
I'm not sure why the price is different on the actual MT4 chart(s), but for now, as far as I'm concerned, due to the fact that I can only go by my broker's pricing, the price on MT4's charts are off by about one pip to the downside. In other words, the 1 to 1.1 pip difference means my brokers price is higher by 1 to 1.1 pips. Also, my other charting platform, which does show my broker's price on the chart without fail, is the same price as the price on the MT4 Market Watch window's Tick Chart tab.
I'm saying all that to say I feel your pain about the price difference between the Market Watch window and the price on the actual chart(s) in MT4. But I think I can work through it, especially if I enter trades with a specific price in mind.
This behaviour is generic and I do not understand why it is ignored in documentation, and never explained by MQ representative anywhere.
In short, brokers may deliver different ask-bid pairs for particular account types, while they deliver always the same chart for all their account types.
From my experience, brokers simply add or subtract a constant, in most cases the same value for both ask and bid. So if you find the two constants, you can use them in your calculations.