MT4 Charts are built by Bid Price

First of all, it is important to know the following facts:

The MetaTrader 4 charts are built with Bid price.
Buy orders are opened by Ask price, but closed by Bid price.
Sell orders are opened by Bid price, but closed by Ask price.

Hence, if traders only look at the chart (Bid price) without regarding the actual Ask Price, then it is likely that they will come into confusing scenarios when Ask price should be used:-

Opening long (buy) positions with Ask price
Closing short (sell) positions with Ask price

The sections below further explains all possible confusing scenarios which traders may encounter.

Reminder: These are not technical issue.

Buy Stop triggered early

If you have a pending order of Buy Stop, you may observe that the Buy Stop is triggered before the bar reaches your Buy Stop level.

Apparently, the Bid price has not yet hit the Buy Stop level.

However, given the fact that Buy orders are opened by Ask price, the Ask price has already hit the Buy Stop level, hence triggered.

Buy Limit triggered late

If you have a pending order of Buy Limit, you may observe the bar is already fall below your Buy Limit level, but the Buy Limit is still not yet triggered.

Apparently, the Bid price has already fall below the Buy Limit level.

However, given the fact that Buy orders are opened by Bid price, the Bid price has not hit the Buy Limit level yet, hence not triggered.

Sell order Take Profit (TP) triggered late

If you have a Sell order and you already set a TP, you may observe the bar is already fall below your TP level, but the TP is still not yet triggered.