Sea salt is ~98% NaCl. The remaining percent is small amounts of calcium sulfate (gypsum), magnesium chloride, calcium chloride, sodium sulfate, etc. The gypsum and calcium chloride can both reduce pH. I don't believe there is [virtually] any sodium bicarbonate in unadulterated sea salt.
However, 4.1 to 3.5 is a big drop, so assuming the meter to be correct, perhaps there is something else going on like CO2 coming out of solution and carbonic acid.