I always thought DS1 was slow to start talking but according to your info he has always been exactly where he should be for his age, possibly ahead now he is 4.

DS2 has been speaking in sentences for ages now. Was saying things like "look mummy, it is a brachiosaurus" or " Come on Byron, lets jump on the trampoline" well before he turned 2. Repeats anything that you ask him to say without a problem, sings whole songs which are clearly recognisable by their tune too. I also put it down to having his older brother talking to him constantly. I guess it will be interesting to see if there is a difference between the 2 of them at school and later in life. Personally I don't think DS2 is brighter than DS1, he just speaks better and because it is such an obvious skill it makes him seem brighter.