Indoor plants acidic soil
That's a super low pH, and I kinda doubt it's real. If you used a cheap home test, it's more likely analytic error (not necessarily your fault, it's just a hard thing to measure) than a real result. 4.0 pH is lower than most plants can tolerate--it's in the range of vinegar. Most plants (I have no experience with this species) prefer a slightly acidic soil, ~6.8pH. So, if your pH is really 4.0, then that's almost certainly a problem. It messes up the cation-exchange capacity of the soil, so even if you fertilize, the plant can't access the nutrients. 

You can easily amend with some lime--if you don't want to dig it up, you can just scratch it into the top of the soil, and water through it. It will leech down through the pot. Hope that helps.