The grant is being cut from Â£6.7m to Â£5m by 2020, but most of S4C's funding now comes from the BBC licence fee.
The debate, in the Commons, will be led Carmarthen West and South Pembrokeshire Conservative MP Simon Hart.
In December, Broadcasting Minister Ed Vaizey said S4C was well funded, with a "guaranteed income" of Â£90m a year.

The action will take place on 25 January and 1 February and affect Neath Port Talbot Hospital, Morriston in Swansea and Princess of Wales in Bridgend.
Unison said non-emergency surgery was "likely to be severely disrupted".
The health board said it was fully committed to resolving concerns.
Unison said affected members at Abertawe Bro Morgannwg University Health Board wanted "pay parity with colleagues doing the same job in other hospitals in Wales".
Earlier this month, 54 staff members voted in favour of taking the industrial action, with two voting no, the turnout for the ballot was 78.9%.
Mark Turner, Unison organiser for ABMU, added: "They are highly skilled technicians of many years training yet are treated as though they have just walked in off the street.
"They are extremely disappointed that after two years of discussions the health board continues to ignore their very reasonable claim in the hope they will go away."
The health board said all staff were valued and it had been working with the trade union to develop a new job description.
The board said the current service needed a workforce with a mixture of skills at different bands.
"We are sorry our continuing discussions with Unison have not been able to resolve our staff's concerns," a spokesman added.
"We remain fully committed to continuing our partnership work with Unison to resolve these concerns and have offered to involve ACAS to assist with this issue.
"In the meantime, we would like to reassure patients should strike action go ahead we will be working to ensure any disruption to services are minimal."
A similar dispute involving X-ray department workers has been resolved.

The Assembly Rooms, which opened in 1977, was badly damaged in a huge blaze that broke out in its roof car park last March.
The city council said rebuilding would be too expensive and the venue would be sold to a private company and could be knocked down and the site redeveloped.
As well as live music, the Assembly Rooms also hosted British Open snooker.
Perhaps its most famous musical moment was in 1983, when Morrissey was hit in the eye with a flower during a Smiths gig and fans invaded the stage at the end.
The venue also hosted performances from acts such as The Clash, Paul Weller and Manic Street Preachers and was home to the city's pantomime each year.
City council Labour leader Ranjit Banwait said he hoped the venue would be replaced with "some kind of cultural offer".
The opposition Conservative group said the local authority had dithered over the plans.
Councillor Robin Wood said the venue could have been kept "alive" in the interim until a bidder was found.
He said: "For over a year now we have been wondering what on earth is going to happen. Now we are told the wrecking ball is going to go at it.
"I think it would be much better to refurbish it and reopen it and look at other alternatives. What we will end up with is a pile of rubble and nothing."
Dave Parry, of the Derby Civic Society, said the venue was too big for the market place and never really achieved what it set out to do.
"We would like to see something that would be representative of the city," he said.
"I'm not sure that it is the right place for a cultural thing, but we also don't want to see a supermarket or block of flats there."
MPs will debate later how Welsh language TV channel S4C should be funded in the future, as its grant from UK ministers continues to shrink.
[NEXT_CONCEPT]
Staff in the sterilisation and disinfection units at some hospitals in south Wales will go on strike for two days over equal pay.
[NEXT_CONCEPT]
The curtain is set to fall on a music venue in Derby where a host of famous acts have thrilled fans for decades.