The fifth MP, Rebekha Sharkie, is part of minor party Centre Alliance.
Ms Gallagher referred herself to the High Court after questions were raised about whether she was eligible to sit in Parliament, but she always maintained she took all reasonable steps to renounce her foreign status before the closure of nominations for the 2016 federal poll, despite the Commonwealth claiming she left her run too late.
"Consequently, I will resign from the Australian Parliament today and seek re-election at the forthcoming by-election, which I believe the earliest can be the 16th of June and I would hope it would be that date".
Gallagher and lower house MPs Justine Keay, Josh Wilson and Susan Lamb are members of the opposition Labor party.
Gallagher's seat in the Senate will be taken by David Smith, the second Labor candidate for the Australian Capital Territory.
"We all knew what the circumstance was last October", when the court ruled on the case of the Nationals' Matt Canavan, Porter said. Shorten said Labor would now consider the implications of the decision.
"If Labor was to lose that, it'd be a huge fillip for Turnbull", Economou said.
Turkey Warns of New Cross-Border Offensive in Syria
Before that, he served as prime minister, from 2003 to 2014. " Turkey will be even more attractive to investors", Erdogan said. Meanwhile, the military also said that the number of terrorists neutralized in Operation Olive Branch has reached 4,403.
The resignations were sparked by the High Court's ruling on Wednesday that Labor senator Katy Gallagher's was also ineligible to stand at the 2016 election.
The four MPs stepping down today, who were also United Kingdom citizens at the time they nominated as candidates in 2016, but had lodged the initial paperwork to renounce it, were making the same failed argument as Gallagher.
Lamb, who holds the highly marginal Queensland seat of Longman will have to renounce her British citizenship before she can recontest her seat.
"Here they have done so by establishing how the law operates in the context of someone in Katy Gallagher's position".
The court ruled unanimously on Wednesday that the senator was disqualified.
"The High Court has adopted the strictest possible reading", George Williams, professor and Dean of Law at the University of New South Wales in Sydney, said. But the Home Office didn't register that she was no longer British until August 16.
That would require an "insurmountable obstacle" to renouncing foreign citizenship, five justices said in a joint decision.