After 33 days of hearing evidence, judge Rohini Walgama told a packed court house that Vidanapathirana, a member of President Mahinda Rajapakse's Freedom Alliance, and three others were guilty of all the charges. Two others were discharged.
All six of them were indicted in December after repeated allegations that there was an unnecessary delay in bringing the case to court, leading to suspicions they were being shielded by their political connections.
Britain's Prime Minister David Cameron had cited the murder of Shaikh, who was beaten then shot in the head, as an example of Colombo's failure to deliver justice.
Shaikh was holidaying in Sri Lanka during a break from his job in the Palestinian territory of Gaza where he was fitting prosthetic limbs.
Hotel manager Tharanga Peiris told the court earlier this year that she tried to stop him going to the aid of another hotel guest who was being beaten by the politician and his gang.
Peiris, manager of the Nature Resort in southern Sri Lanka, said she had a clear view of Vidanapathirana and his men during the attack.
Another hotel guest said that the attackers had "behaved like animals" and went on a rampage smashing plates, glasses, car windows and toppling a three-wheel taxi parked outside.
Shaikh's girlfriend, who was knocked unconscious after a brutal assault, returned to Sri Lanka this year to give evidence and identified one of her attackers in court.
She was not seen on Friday, but Shaikh's brother Nasir listened to the judgement being delivered in a packed courtroom.The case was initially heard in the southern town of Tangalle, the hometown of the main accused, but shifted to the capital Colombo last year following allegations that the suspects were intimidating witnesses.
"There was a lot of media attention on this case," judge Walgama said while sentencing the four men. "This is an important case in our legal history and I was able to conclude the trial in just over three months of hearings."
It was not immediately clear if the men would appeal the verdict and the 20-year jail sentences. Two more accused in the case were discharged.