What's the size of the file?

GCode for UCCNC is just an ASCII enocded text file - it's a pretty shit way of storing data efficiently, but it does the job for most applications.

Notice when you load a file UCCNC shows you the entire toolpath immediately and shows you each line in the scroll box. That implies it's probably loading the file into memory and also creating a load more associated bits of memory to display it to you in a nice format (each one of those toolpath lines on the diagram for example needs some associated variables). This is great from a user experience point of view, but pretty limiting when wanting to handle large files - when you watch a movie for example, the program will buffer a small amount at a time rather than pull multiple GB into RAM. Let's guestimate that UCCNC uses 4x the file size in RAM when you load it up.

Now how much of your 4GB is free after Windows and UCCNC is loaded? Maybe 2GB? So any file over 500Mb is going to cause problems.

As A Camera says, more RAM will probably help you out here, and an SSD will make the loading of the file quicker.