Lorenzo and his 2012 Honda adversary Dani Pedrosa have between them topped every day so far in testing, but Rossi and Marquez have often finished within half a second of their respective team-mates.
Lorenzo believes such pace points to all four men being title contenders in 2013.
"Over the six days at Sepang, Dani and I were the top two riders," Lorenzo told the official MotoGP website.
"At the same time, Marquez and Valentino were both really, really close.
"The reality is that it's between four favourites for the world championship and it looks very equal.
"We're all on a similar level, so it's going to be a hard season for all of the top riders in the class."
While Lorenzo was at one stage more than three tenths of a second clear of the field in Malaysia, he cautioned that it will not be until the forthcoming Austin test that Yamaha can gauge just how much progress it has made over the winter.
"I feel very strong on the bike, which is actually feeling better than last year," he said.
"Having said that, that was just one track so we'll have to see what happens on circuits with different characteristics because every track is a different world.
"We don't have any reference [at Austin] so we'll just try and learn the track and improve the set-up little by little."
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