Harriet Harman, George Young, Lynne Featherstone and Adam Price on the calls for electoral reform
The parties have been arguing over whether electoral reform is necessary to rebuild public trust in Parliament.
Harriet Harman told the Daily Politics that Labour have not broken a pledge on voting reform in the 1990s and were now sincere about proposed changes.
Lib Dem Lynne Featherstone said it was "glaringly clear" the current system was unfair and Plaid Cymru's Adam Price said smaller parties were penalised.
Sir George Young said the Tories were "committed" to the existing system.
Labour has pledged to hold a referendum on a new voting system, if elected, a position backed by the Lib Dems but opposed by the Tories.
Asked whether the Tories would be prepared to contemplate reform as part of a post-election coalition deal with the Lib Dems, Sir George said the party had a "clear commitment" to the current first-past-the-post system.
In a special election edition of the BBC show, the party representatives each set out their arguments for reform of Parliament, the conduct of politics in general and the representation of women and ethnic minorities at Westminster.
All four accepted that the expenses scandal had done real damage to trust in politicians.
Ms Harman, deputy Labour leader, said she had been "dismayed" by the saga but added: "I want to assure people that the expenses system is completely sorted".
Sir George Young, shadow leader of the House of Commons. said public anger over the affair was justified.
But he believed "disillusionment" with politics ran deeper and parliament needed to change the way it does business to make it more relevant and to give MPs more power.
"I hope that the next parliament can put the disaster of the last one behind it and start with a clean sheet," he said.
"We want your MP to be more effective in a more effective House of Commons".
He said his party supported tighter controls on lobbying by former ministers while Ms Harman said Labour had acted to require MPs to disclose information about second jobs.
For the Lib Dems, Ms Featherstone said Parliament had been "found out" by the scandal, describing it as "the tip of the iceberg of a rotten and corrupted politics system".
"You cannot rebuild trust in politics with warm words and minor changes. We need fundamental political reform," she said.
Reform to the voting system, to make it more proportionate, would end the current situation where "80 marginals" may determine the outcome of the election.
She said Parliament must improve the way it is perceived by the public.
"Prime minister's questions may be good blood sport but it is absolutely rubbish in terms of demonstrating how we behave in the House," she said.
Plaid Cymru's Adam Price - who stood down as an MP last month - said Thursday's election marked a chance to end a "decade of dishonesty in British politics".
Also urging future voting reform, he called for a "rainbow Parliament" with increased representation for smaller parties and an end to parties having "absolute majorities".
"Proportional representation works for us in Scotland and Wales because we can get more voices," he said.
The parties clashed over their campaign tactics following a recent over Labour leaflets suggesting the Tories would scrap guarantees for cancer patients.
Sir George said this had brought "politics into disrepute" but Ms Harman defended the mail shot - sent to thousands of people.
"I think people have a right to know what is at stake and I don't think that's scaremongering."
All four agreed they must do more to make Westminster more representative in terms of the number of female and ethnic minority MPs.
Under David Cameron's leadership, Sir George said the Tories had "transformed" the make-up of its party while the Lib Dems said they were working hard to become more diverse.
"We are streets ahead of other parties but I have always recognised we have further to go," Ms Harman added.
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