"LOOK AT ALL THIS MONEY WE HAVE!!"
"Should we make more episodes?"
"Don't be ridiculous, 12 episodes is the anime standard!"
"So what should we do with the extra budget money?"
"What episode are we at again?"
"Okay, for the next one, CHANGE THE OPENING THEME!!"
"What about the visuals"
"Do we have enough money to change those too?"
"Well those can stay the same."
And that is why something like that happens, there's no meaning behind it, they do it because they can.
To be honest, there might have been a disagreement from the start about the opening theme and they just decided halfway to switch, or maybe the majority didn't like the first theme so they decided to try to do what was best for the anime. All this to say, it has nothing to do with the anime itself, I watched it too, and nothing really changed from episode 4 to 5.