My laptop now has 24 GB of RAM and is a real smooth editing machine. Of course it has a fast 4-core hyperthreaded CPU and two SSD drives, all are required to edit AVCHD satisfactorily.
Premiere takes far more ram now due to the GPU acceleration and changes in memory management. 4GB is far to low. 8GB is bare minimum for base workflows and 16GB for most 1080+ work currently.