The fifth MP, Rebekha Sharkie, is part of minor party Centre Alliance.
Josh Wilsonhas resigned from Federal Parliament this morning after being caught up in the dual citizenship scandal, but has said he will contest a by-election.
In a statement, Sharkie said Gallagher's case had led her to take new advice.
Three Labor MPs and one crossbencher have resigned from Parliament over dual citizenship, setting up a Saturday byelection bonanza with voters to go to the polls in four states.
"If Labor was to lose that, it'd be a huge fillip for Turnbull", Economou said.
Gallagher was aware of her British passport as her father was born in England, so the High Court ruled that she had not taken "all reasonable steps" to relinquish her British links.
Earlier in the day, the High Court determined that the Labor senator was ineligible for Parliament because the defence of taking "all steps reasonably required" did not apply to her.
Cavs not disciplining Rodney Hood for not entering Game 4
Success! An email has been sent with a link to confirm list signup. "Instead, Hood refused to go in, multiple sources confirmed". He averaged just 10.8 points in 25.3 minutes per game over 21 regular-season games with the Cavs.
Asked if Ms Lamb had retrieved the documents, Mr Shorten said: "I am not aware of each piece of paper that she has".
Australian Attorney General Christian Porter stressed that the ineligible lawmakers should have quit in October, when the High Court ruled that dual nationals could only be elected if a foreign law made it insurmountable to renounce a second citizenship.
"It is a crystal clear clarification of the law as it was stated in the [Matt] Canavan decision past year, and anyone, Bill Shorten or anyone else, who says this is a reinterpretation or a change, is talking absolute rubbish", he said. But the Commonwealth says that she was still too late, as the UK Home Office did not registered her renunciation until August, more than a month after the election.
As a senator, Gallagher's disqualification does not trigger a byelection - she is set to be replaced on a recount by the next person on the ALP ticket, David Smith.
"Those four people must resign, they must resign today", Porter told reporters following the court decision and hours before the resignations were announced.
That would require an "insurmountable obstacle" to renouncing foreign citizenship, five justices said in a joint decision.