4 times a week isn't use, it's closer to abuse. People who drink more than a beer or two that often would probably be worse off.
Further, that sample of people who smoked "four or more marijuana cigarettes a week" could include people who had daily indoor bonfires with the stuff; it makes only an arbitrary distinction between amounts smoked and their corellation to test performance. It's pseudo-research, research aimed not at finding truth but rather at getting more grant money from the grantor (probably the money machine behind the DEA, NIDA & Partnership) by finding the results they want to find. It's fairly prevalent; research science is one of the least profitable fields an educated person can go into.