B.,
I know what you mean about finding different things for little brothers, I have 5 boys and some years it is so hard to find original toys. Some things my boys liked when they were around 2 were the toy farm tractors. We always got the high quality ones from the farm stores, TSC (tractor supply co), or Quality Home and Farm. We covered many years with those. Buying different types of tractors, combines, etc, and also buying implements to go with them such as planters, disc, hay mowers, rakes etc.
For your 2 year old I would recommend the smallest simplest one you can find. Maybe get a similiar one for your 4 year old too. My oldest will be 13 in less than a month and my little boys are STILL playing with some of them.
Another great toy that I found that my kids seem to love are called WEDGITS they are a type of diamond shaped building block. We have quite a collection of these now (a big 20 gallon tub full) When they are little they can just stack them or sort them into different colors. But they do come with building ideas (some are quite complicated) And my oldest boys 13 and 10 will still play with them, as it is still challenging to them, but my 18 month old likes to play with them too. Every so often for someones birthday or for christmas we will get another set and add them to the tub.
Thats another thing we do, with other building sets, like magnetix,(although your kids are still a bit young for this) we put everyones set together in a tub, and will buy a new set as a gift and add it to the main set. That way all the kids can play at the same time or if just one boy is playing they can build huge things. We also have done this with Lincoln Logs, Legos (duplo and regular), k-nex, and tinker toys. That way you can get nice toys, not spend a lot a once but keep building to some large play sets.
I hope this was helpful to you.