Mega Man World 4 continues the Blue Bomber's Game Boy saga, and while this game continues on the same formula established by the previous Game Boy games, there is some new stuff added.

The biggest addition, I'd say, is the addition of P Chips. P Chips can drop from enemies and can also be found in stages. These are basically currency for the "shop" Dr. Light has in the game! This is actually pretty cool, and predates Mega Man 7's usage of bolts as currency by about 2 years.
You can't buy much in the store, but what is there is very useful, like E-Tanks and the Energy Balancer item, which I did get.

In addition, production values seem higher for this one. There are more cutscenes and cool animations to see in this one, and even some dialogue close to the end of the game, something that didn't really happen in the previous World games. The soundtrack is also pretty solid, with great renditions of the songs found in the NES titles, plus some new ones.

Besides that, you go through 4 stages from Mega Man 4 Robot Masters, and after a boss, you go through 4 stages from Mega Man 5 Robot Masters. I'd say that the level design in this one is my favourite in the World games. There are familiar set-pieces, for those who played the NES games, but there are also new gimmicks added, that make the stages pretty fun.

The new major Robot Master, Ballade, is also pretty solid, and on-par with Mega Man World 3's Punk, in my opinion. Although I did find the last Wily stage to be kind of long, this time around. And this game actually brings back the traditional Boss Rush from the NES games, which doesn't help with the length.

Regardless, so far, I think Mega Man World 4 is my favourite of the Game Boy Mega Man titles, with solid level design and great production values!