You are asking about how cellular voice and data differ. They do so that's the quick answer.
As to improving this, I think you are using this indoors and cellular often has trouble indoors. Try moving the hotspot device to window sills around the home. I owned a few of these and this helped.
As to 3G/4G it really doesn't matter. You can have issues on either.
I have a general connectivity problem where I stay. Looking for alternative mobile service provider, I managed to get a Vodafone guy to come to our place to check the signal strength inside the premise.
Voice seems to work fine. But, my more pressing need is data. Their dongle on my laptop as well as test mobile hotspot to my laptop failed to give good data connectivity. My laptop was showing excellent strength to the hotspot, but hotspot-to-tower was problematic.
I am curious to know why there is a mismatch between voice and data signal strength. The sim card in the dongle and test mobile were 3G. We do not have 4G here.