If the file is 3.8 GB and you computer has 4 GB of RAM, I would say it is almost a miracle the project opens at all.
Getting more RAM would definitely be my first thing to try.
I suppose you are using 64-bit ArchiCAD as well. The 32-bit version of ArchiCAD can use only 2 GB of RAM.
I would probably go with 16 GB of RAM with such a huge project size.
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