It'll take more gas than your terrorists can afford.
Some rough calculations: This Wikipedia article (sorry) says tetrafluoromethane is a 6500 times more powerful greenhouse than carbon dioxide. Scientific American cites a CO2 concentration of 30,000 ppm as necessary to cause the "Venus effect" you mentioned. Not totally confident on the chemistry here, but those numbers give a atmospheric concentration of 4.6 ppm required to Venus-ify earth.
If we use a number of 10^20 moles of gas in earth's atmosphere (calculation), that's roughly 10^14 moles of gas, or (again roughly) 10^15 liters of gas. That's one quadrillion liters. Using this price of 1.6 USD / kg wholesale, your terrorist organization would be paying on the order of a few times the United States annual GDP to buy all this gas. Unless your terrorist organization is the United States (plot twist), this world destruction scheme is pretty infeasible.
Your terrorists should probably pick another approach, like blowing a hole in the ozone layer. Intuitively, this makes sense. If the world's industrial activities haven't been able to boil off the oceans, a midsized terrorist organization is probably gonna find the task pretty tough.
Edit: ozone concentration monitoring is surprisingly limited in the open ocean. Your terrorists could conceivably remain undetected for years if they chose to release CFCs in the middle of the pacific. You would definitely need less CFCs than tetrafluoromethane to cause damage to earth's biosphere, but I couldn't say exactly how much.