Who knows 
How long had the Pleco been in the tank? It could have died from simply being shocked with the move from the store to your tank. Also, 4 gallons is not very much....you could easily overdo Co2 if you are not carefull. But......an airstone is not very good at getting Co2 into the water. Was there also air being fed into the airstone? If so, little to no Co2 was probably staying in the tank. It was all being rushed to the surface where it dissipated. I think that maybe he died of stress or simply being trapped. Or he had nothing to eat? Sorry for all the random thoughts.
How long had the Pleco been in the tank? It could have died from simply being shocked with the move from the store to your tank. Also, 4 gallons is not very much....you could easily overdo Co2 if you are not carefull. But......an airstone is not very good at getting Co2 into the water. Was there also air being fed into the airstone? If so, little to no Co2 was probably staying in the tank. It was all being rushed to the surface where it dissipated. I think that maybe he died of stress or simply being trapped. Or he had nothing to eat? Sorry for all the random thoughts.
