Hi Valerie,
The Ledipasvir in Harvoni needs stomach acids to be effectively absorbed. Magnesium, which is also found in many antacids, acts to reduce stomach acidity. So if you are on Harvoni (Sofosbuvir/Ledipasvir) then it is advisable to avoid taking antacids or magnesium supplements at least around the same time as your medication. I can't advise whether taking them at different times of day would make a difference.
The above may be a reason why your viral load has not decreased as much as expected at 4 weeks but I'm not a doctor so you need to discuss this with one who can provide expert advice.