Australian pace spearhead Mitchell Johnson is uncertain for the fourth and final cricket Test against India in Sydney, due to a hamstring problem.It forced him to skip a practice session in the city on Saturday.According to media reports, Johnson complained of hamstring soreness and decided not to travel with the team for their first practice at the nets in Sydney. If the bowler, who was slightly off-colour in the drawn third Test, fails to be match-ready, Mitchell Starc or Peter Siddle could be brought in to the side. The Sydney Test begins on January 6. Australia lead the series 2-Nil.