Yes, it is harder. Scarier. It's a lot deeper than it's predecessors, and HGSS. You just have to try harder ;)
I disagree- manly because she says she has been playing Emerald. Before I got Pearl and Platinum, Emerald was all I played, and I still restart and play it often now to transfer useful TMs via Pal Park, but I find it much harder. Obviously, this is just my opinion, but allow me to elaborate:
It at least SEEMS that skills that sometimes paralyze/burn/poison (meaning they do damage and theres like 10-30% chance of additonal effects) suceed in doing just that alot more often on Emerald. I also find that all of my Pokémon flinch a helluva lot more on Emerald, and the enemies seem to be able to crit me at least 50% more often than either I can crit them or they crit me on gen 4 games.
I also find that money is harder to come by- by the time I get to the E4 first time round on Emerald, I have very little money even though I buy maybe 10 Pokéballs at the start and never buy any again because I know who I wanna catch for my team and don't bother catching anyone else. I also buy little else in game, but when I get to E4/Victory Road and it's time to stock up on supplies, it leaves me almost empty of cash.
Lastly, I find the Elite Four much much harder also. Obviously they start at a lower level in Emerald (I think the lowest is 46), so my Pokémon are lower also, but on the current saved game I have, I have yet to beat them, or even get as far as the Champion without losing all my Pokémon and 'Whiting out' several times. This hasn't happened to me since I first started playing with Pokémon blue all those years ago, and even then, I don't think it happened as often as it does on Emerald. Now, all these years on, I have experience on my side, a knowledge and understanding of the Pokémon, their weakness and what moves do what- not to mention a team that has been handpicked to be the best (types-wise) for beating the E4 and champ, yet I fail several times before I succeed.
Maybe it is just me going wrong, but I find that the Gen 4 storyline much easier and supportive as it gets your Pokémon to a decent level to face the E4 and champ at the end. I agree that the Gen 4 games seem scarier, and they are longer also, but I think this helps.
So, long story short- in MY opinion, Emerald is much harder than Pearl/Platinum.