Nicola Sturgeon has challenged Ed Miliband to form a "progressive alliance" to prevent David Cameron forming a Conservative-led government.
The SNP leader said Labour would not be bold enough on its own to defeat the Tories and urged Miliband to "kick out" David Cameron.
"We have a chance to kick out David Cameron out of Downing Street, don't turn your back on it. The people will never forgive you," Sturgeon said to Miliband.
"I will work with Labour, with Leanne [Wood, the leader of Plaid Cymru], with Natalie [Bennett, of the Greens] to get rid of the Tories," she added.
The pair earlier disagreed on the issue of Trident, with Sturgeon saying the SNP would instead spend the £100bn required to renew the nuclear arsenal on "childcare, health and education".
Miliband, meanwhile, backed Trident and as the debate veered towards foreign policy, said he would continue to launch air strikes on Isis.
Earlier in the night, Ukip leader Nigel Farage drew boos from the BBC audience when he disputed its political balance.
Farage, who had already been jeered by some audience members when he mentioned immigration, said to presenter David Dimbleby: "This is left field audience, even by the standards of the BBC."
He was also unafraid of repeating the HIV comment he used at the previous leaders' debate when arguing how immigrants' health tourism had blighted the NHS.
In one outburst, Farage berated Miliband after the Labour leader suggested his Ukip counterpart would transform the NHS into a US-style health insurance system.
"Stop lying! Ukip believes in an NHS that is free at the point of delivery," Farage responded angrily.
What the leaders said:
"The problem I have with you Nigel is you want to exploit people's fears, not address them"
"Everything I do as Prime Minister, I will put working families first"
"I'm the son of immigrants and I've recognised Labour needed to change"
"Isis is a disgusting, evil organisation and we must be them at home and abroad"
" The last Labour government did not build enough homes. Large developers have a stranglehold on the market."
"In England, under this government, we are building fewer homes than at any time since the 1920s. It is a massive, massive problem."
"We need to replace the Tories with something better."
"We have a chance to kick out David Cameron out of Downing Street, don't turn your back on it. The people will never forgive you"
"Austerity means that by 2020 there will be one million more children living in poverty. That would be a betrayal of a future generation."
"If austerity is d more people into poverty and undermining services, then it is time to change"
"I will work with Labour, with Leanne, with Natalie to get rid of the Tories"
"I'll fight for ordinary folk against the corporate political classes. I'm unafraid to do so"
"I believe the gap between the professional political class and ordinary people has never been wider"
"Stop lying! Ukip believes in an NHS that is free at the point of delivery"
"This general election has become farcical. Parties are trying to bribe you with borrowed money"
"English voters are concerned about SNP tail wagging the Labour dog. It can't be allowed to happen"
"This is the world's sixth richest country. We can fund proper education and free health and social care for elderly"
"Vote for what you believe in, for now and future generations to come. If you want real change, you have to vote for it"
"Let's increase aid, increase diplomatic efforts - that's how we make the world safer"
"We have huge problem in that houses have become financial assets. We want to build 500,000 affordable homes by 2020"
"I want to celebrate the contribution of migrants to Britain and believe we should all do that"
"Trident nuclear weapons do not make me feel any safer...they are hideous weapons of mass destruction"
"We want to invest in future and job creation. Continuing on the path of austerity will deliver an uncertain future"
"'I will not prop up a Tory government, neither will I prop up a Labour government hell bent on implementing Tory policies"
"Government spending should not be thought of as expenditure but as investment in the future"
"Plaid Cymru does not accept that this is as good as it gets, we can take a different course"
"Plaid Cymru will fight to ensure that Wales' financial disadvantage is ended"
"We can't justify spending £100bn on weapons of mass destruction in the form of Trident"