My girls are 19 months and just about 4 years.
They get along REALLY well for the most part - my youngest wants to do what the older one is doing, so most of the whining or arguing is about DD2 trying to "get" whatever DD1 has (and dd2 is FAST!)
My oldest is actually very thoughtful so she will often bring something to my younger daughter to help distract her. She'll also call me for help. The issues come up with my less-verbal toddler yanks her hair out of frustration