Program types in MQL4

First you need to decide which type of program you are going to create. Content and functionality of the program will be entirely designed according to this desicion. With MQL4 following three program types can be created;

Expert Advisor; This type of program is designed to call in every tick event in MQL and it runs from beginning in tick event. There is not any pre defined expert advisor in MetaTrader default, you need to make it.

Script; This type of program is designed to call only once. It does not provide continuous control on trading. Similarly to expert advisor, there is not any script default, user needs to create this type.

Custom Indicator; This type of program is also called in evey single tick event but it has no ability to trade. Its purpose is to analyse market in graphical interface so that user analyse market visually. There are two types of indicators technical and custom indicators. There are some indicators in MetaTrader default but also user can make his.

According to purpose one of the program type must be choosen before start to coding.

Simultaneous Use

It is possible to use only one expert advisor in a parity in same screen. It is forbidden to use more than one. 

Script files also can be used only one at same time in a parity. It is also forbidden to use more than one at the same time.

Custom indicator programs can be used more than one in same parity and they do not effect each other.