How to install MT4
You need to download a distributive and launch it on your computer. Next you need to choose a destination folder and continue the installation. After installation the terminal will launch automatically.
You can also install the terminal without deleting the previous version. In that case all the settings of your terminal will remain the same. Although, the exception is possible: profile and DEFAULT template, and standard MQL4-programms, included in the distributive could be changed. That's why it is not recommended to change standard templates and profiles. As for MQL4-programmes - you should create them with unique names.
If you trade on several accounts simultaneously you should install the corresponding number of terminals to different folders. Every terminal has its individual settings with the exception of registration data and interface language which are recorded into the system register.