I would repace the original 4GB with upgrade memory, test to see if it boots, and then put original memory into upgrade slot and test, if it fails, that slot is probably bad. You could use it in the other bank, won't be as fast, but you'll have 8GB.
After any memory upgrade or change, I suggest running a couple of passes of MemTest86, the free version, to ensure all is well.
MemTest86
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