Interesting. Air is approx 1 kg/m^3, so that tank is 0.4 kg at 1 bar, and a typical COPV can go to say 300 bar, so we're talking 120 kg.
For a 1000 kg car and with 200 m/s nozzle exit velocity, to hover the car you need (with m_0 g = m_a v/t) about 50 kg of air per second, so we're talking about roughly 2.5 seconds of flight (from those 400 litres).
For a 1000 kg car and with 200 m/s nozzle exit velocity, to hover the car you need (with m_0 g = m_a v/t) about 50 kg of air per second, so we're talking about roughly 2.5 seconds of flight (from those 400 litres).