Most of these creatures really aren't anything that special.They're hardly monsters that only seasoned adventurers can deal with, they're the CR they're at for a reason. There's still plenty of high CR creatures that can feasibly threaten tier 2 and 3 PCs in the monster manual and other 5e books which can definitely give players a real sense of progression.
The dragon is also kind of a bad example here. The reason they're introducing you to dragons early is because they're plot relevant and so it makes sense that one would show up early. You can't actually fight the dragon either. You can kill it, but only by going with the extremely cheesy barrelmancy strategy of sneaking in without triggering the cutscene, placing a very large amount of explosive barrels next to it, and detonating them to deal enough guaranteed damage to kill it. I'm not sure if this strategy even works anymore though since they've nerfed barrelmancy a lot since the EA started.
Needless to say, this isn't really a fair fight in any stretch. An actual fair fight against the dragon would be nearly impossible since it's a CR 17 Adult red dragon that can knock out multiple level 4 PCs in a single turn and can instantly kill an entire party of level 4 PCs with it's breath weapon. A party of 4 PCs can totally kill it in an actual fight at level 13 or so, but it's definitely not something you can really do at level 4.
