4 Gears but can only switch to 3

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Hey, could anybody help me?
So when I turn the bike on it says there are 4 different speed modes, but on the buttons I can only switch between 1, 2 and 3 and I can't find how to switch to 4th.
Thanks

 

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Model -- MilG , on the receipt it says Blue Vespa
(Also can't find link if anybody could help it would be apprecisted)