Scottish champions Celtic kept their quadruple trophy hunt alive despite suffering the affects of a comedown after scaling the heights of the UEFA Cup in midweek.

After the euphoria of beating Stuttgart on Thursday night, the supporters stayed away while Martin O'Neill made 10 changes to his side.

They must have known what was coming with Celtic failing to find the spark that lit up Parkhead on Thursday.

It was only a first-half John Hartson penalty and late goals from Jamie Smith and the Welshman which put them into the fifth round of the Tennent's Scottish Cup.

The attendance of 26,205 was even lower than expected and that lifted the hopes of First Division St Johnstone manager Billy Stark.

But Celtic's second string struggled to get going, although those Hoops fans who had shown up almost had something to celebrate in the fourth minute.

Stephen Crainey exchanged passes with Steve Guppy and he threaded the ball through to Momo Sylla, but his control let him down and Kevin Cuthbert collected.

Moments later David Fernandez almost got on the end of Hartson's pass, but David McClune made a vital interception and clearance.

It looked like debutant Javier Sanchez Broto would have a quiet debut, but he made a great save in the ninth minute to deny the visitors a shock lead.

Johan Mjallby gave away a foul 25 yards out after fouling Stuart Noble and Paul Hartley hit a low drive from Tommy Lovenkrands' short free-kick which looked destined for the bottom corner, but the Spaniard got down well.

It was not difficult to hear the travelling St Johnstone support and they were lifted by that moment and another chance just four minutes later.

Mjallby, making a quicker-than-expected return after injury, looked sluggish and Paddy Connolly got away from him, but Ulrik Laursen got across to deflect his shot into the sidenetting.

Celtic continued to look uneasy at the back, but Hartson could have calmed their nerves in the 16th minute after being found in space by Colin Healy, but his right-foot strike was comfortably saved Cuthbert.

The home side were finding it difficult to get into their rhythm, but a Healy effort was deflected behind by John Robertson before Hartson screwed his next shot wide.

The Welshman was appealing for a penalty in the 25th minute after going down under the challenge of Darren Dods, but referee Hugh Dallas never looked interested.

Celtic eased their concerns in the 32nd minute when Hartson sent Cuthbert the wrong way from the spot after Healy had been brought down by Robertson in the box.

The visiting players and fans did not agree with the decision and St Johnstone defender was shown a yellow card to add to his misery.

Hartson then picked out Smith in the box and he got away an effort from a tight-angle, but Cuthbert closed him down and deflected the ball behind.

Celtic could have sealed their place in the next round in the very first minute of the second half.

Mjallby picked out Fernandez and he fed Hartson, whose effort with the outside of his foot was superbly touched past the post by Cuthbert.

The home side almost paid in the 51st minute when Lovenkrands picked out Hartley in the centre, but Crainey did enough to prevent him hitting the target.

That woke Celtic again and Sylla's shot from the right of the area bounced just past the upright which Hartson close to getting a touch.

Fernandez produced a great piece of skill to make room for himself in the 72nd minute and he was unfortunate to see his chip go just wide.

Cuthbert did well to tip Smith's dipping 25-yard free-kick over the crossbar before Hartson's deflected effort flew just wide.

But Smith did wrap up their win with 14 minutes left when Smith headed Hartson's cross past Cuthbert from close range.

O'Neill gave 18-year-old goalkeeper David Marshall his first competitive appearance for Celtic for the last eight minutes and Broto left to a standing ovation.

Hartson compounded St Johnstone's misery with five minutes remaining, again scoring from the spot after Shaun Maloney was brought down in the area.

The scoreline was harsh on the Perth side - but Celtic can now look forward to Thursday's trip to Germany in the knowledge that the four trophies are still a possibility.

Teams

Celtic Broto (Marshall 82), Mjallby, Laursen, Crainey, Smith, Sylla, Lambert (Maloney 79), Healy, Guppy, Hartson, Fernandez.

Subs Not Used: McNamara, Boyd, Petrov.

Goals: Hartson 32 pen, Smith 77, Hartson 86 pen.

St Johnstone Cuthbert, Baxter, McClune (Hay 81), Dods, John Robertson, Maxwell, Noble (Panther 30), Hartley, Lovenkrands, Connolly, Parker.

Subs Not Used: Miotto, Ferry, Forsyth.

Booked: John Robertson.

Att: 26,205

Ref: H Dallas (Scotland).