differences among "merely", "just", "really" and "truly"

The only significant difference I'm aware of is in emphasis/intensity. 1) is the least emphatic/intense, 2) is stronger and 3) is the most emphatic/intense.

Yes, intensity.

#1 is by far the most common/ likely.

We also have a useful phrase, "it is on the tip of my tongue", meaning something you know, but cannot easily bring to mind, or easily articulate.

I think they all have different connotations.

Merely is dismissive. I merely couldn't articulate them. It isn't a big deal.
Really implies that it is not what is seems. It might look like I was ignorant, but really I couldn't articulate them.
Truly clarifies what the facts are. You might think I know how to speak, but I truly couldn't articulate the answer.