Ferrari's Fernando Alonso has congratulated Red Bull's Sebastian Vettel on his fourth F1 title but wonders just how great the champion really is compared to those before him. Or, is Alonso just bitter about losing?GREATER NOIDA, India - Ferrari's Fernando Alonso congratulated Red Bull's Sebastian Vettel on his fourth consecutive Formula 1 world title on Sunday but reserved judgement on where the German stands among the greats.Alonso saw his faint chances of keeping the championship battle alive evaporate when Vettel completed the Indian F1 GP hat trick to become its youngest quadruple champion.Alonso, who finished 11th, told reporters: "Congratulations to Sebastian. A very, very strong season, dominating especially the second part of the year with many wins. Well done to him and to the team. I hope we can make things more difficult in 2014."DIFFERENT STROKES?Alonso went into Sunday's race trailing Vettel by 90 points and the German sealed the championship with the ruthlessness that has become the hallmark of his sustained domination in the past four years. The 26-year-old's success has left many wondering if Vettel might be the greatest modern Formula 1 driver but numbers alone would not convince Alonso, who is widely regarded as the sport's most complete driver even if his car is currently inferior.Alonso said: "Different opinions for different people and all of them we need to respect. Many people think Ayrton (Senna) was the best. He won only three championships, other people have won more. Some people think Michael (Schumacher) is the greatest. If you see the numbers, he should be the best because he has 91 victories and seven titles."Vettel's driving performance is big with numbers, especially now being a four-times champion. I remember him winning 14 or 15 races in 2011, in 2013 I don't know many. Now it's 10, he will win maybe 13 races in 2013."(Vettel won five races in 2010 and 2012, 11 in 2011 and 10 (to India on Oct 27) in 2013, the last six consecutive. - Editor)Alonso would not hazard a guess whether he, given an equally fast car, could beat Vettel and preferred to focus on mounting a stiffer challenge in 2014. "I'm not here to guess... today is the day to congratulate him and for him to enjoy the championship, and it's the day for us to start working even harder."Red Bull has been very strong, dominating for the last four years. We need to do something better than what we have because it's clear that it's not enough. We will start immediately."For the time being, Alonso was happy trying to ensure Ferrari finished second in the Constructor's championship after being overtaken by Mercedes. "For me, it makes a difference to have the team happy. They, at the end of the month, pay me... you need to make the person happy who gives you food," he added. Stay with Wheels24 for the 2013 F1 season – fresh reports every day.