A very curious case. An existing system with 4 RAM modules had it’s CPU upgraded. After replacing the CPU and restarting the system, BIOS was beeping 3 short beeps and 1 long beep and it would not continue booting.
Removing modules 3 and 4 and only leaving modules 1 and 2 resulted in normal start-up. First, I thought it was due to modules themselves, then after swapping the modules it turned out only slots 3 and 4 would not work.
In the end, it turned out, the issue was with dirty/greasy probably CPU pins (to note, in my case those were not pins but rather dots – LGA 1155). After cleaning the CPU contacts with alcohol all worked fine.
Lessons 4 Devs 