Napa was sent off for the shot on Mark Percival, which sparked an all-in melee, in the 71st minute of Thursday's 28-8 loss at St Helens.
"Catalans Dragons forward Dylan Napa has been charged with grade D striking," a Rugby Football League statement read.
"Napa pleaded guilty to a lower charge of a reckless high tackle, but the tribunal found him guilty of the original charge.
"Napa has been handed a four-match ban and a £500 fine."
It was a Super League debut to forget for ex-Queensland prop Napa, who signed a two year deal with French club Catalans after the Bulldogs opted not to offer him a new NRL contract.
The former Canterbury and Sydney Roosters front-rower was not the only Dragons player to face the wrath of the disciplinary panel, with fellow forward Gil Dudson to serve a three-game ban.
Both players will be out until at least March.
Dudson was sent to the sin bin for a punch during the opening minute of last week's fixture.
"Catalans Dragons forward Gil Dudson has pleaded guilty to a grade D charge of punching in their recent fixture with St Helens, and has received a three-match suspension with a £500 fine," the RFL statement said.
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