Update your post to include full laptop hardware specs and OS information.
Windows 10? Look in Reliability History and Event Viewer for error codes, warnings, or perhaps even informational events that correspond with the times of crashes and blue screens.
RAM: Specs for the 4 GB soldered in and the 4 GB of RAM that you added.
How did you test the new stick?
Does the new stick match the RAM requirements of the laptop's motherboard?