Metatrader's program folder is where you installed you MT4 terminal to. The default location is under C:\Program Files (x86)\ and this is also the folder where terminal.exe (the main MT4 executable) is located. If the above does not help you in any way, please read the following:

You can quickly locate your mt4 program folder by right-clicking on the icon you use to launch your MT4 and choosing "Open File Location" from the context menu.

This will open your MT4's program folder.

Alternatively, you can follow these 3 short steps to locate the program folder:

1) Choose "Open Data Folder" from Metatrader's File menu, which will open you data folder:

2) In you MT4's data folder you'll see a text file named origin.txt. You need to double click on this file to see the contents.

3) Use notepad to open the origin.txt file - the file's contents will be the full path to your MT4's installation folder. This will look something like: