The main reason this is a bad idea, is the smaller your party size, the longer your queue times for DPS.... you'd think they would have learned this from WoW and AT LEAST went with the 5 person group rather than going with an even lower number.
It's sad but true that Tanking and Healing are both stressful and demanding jobs where it can often feel a lot more like the whole party is riding on you. Don't get me wrong, I've tanked and healed for years in MMOs, and I am healing even now.
I have also done my share of DD as well. As a DD, you only have to worry about 2 things for the most part, keep yourself alive by not standing in the bad, and deal damage.
A tank has to worry about keeping themselves alive, mob positioning, threat, and doing their rotation so that they hold their threat and hopefully using cooldowns at the correct time while still also moving out of the bad, not to mention leading the party in dungeons where you go from one end to the other.
A healer has to worry about, keeping everyone else alive, keeping themselves alive, moving out of the bad, dispelling bad effects, and raising people during an encounter when things go south.
I'm not saying DD is EASY or anything like that, but it is, in general, less stressful than the other two roles for the majority of players. So it stands to reason, that a lot more players are going to gravitate towards DD roles than the tank and healer roles.
In an MMO, a larger party size means a shorter queue for DD, and less healers and tanks needed per DD. For example, WoW's 5 man groups or FFXI's 6 man groups.
Why then, knowing what they know about MMOs in general and the roles that players inevitably gravitate towards, would they choose to willfully go with an even smaller party size than the norm?
A 4 man group set up means that you are actively expecting 50% of the MMO player base to 'suck it up and play a tank or healer' if they want to get decent queue times, or even find groups with FCs and friends who actually have room for them. IMO, this leads to more and more players who do not enjoy tanking or healing feeling pressured into it, and ultimately it turns out bad for the game in the long run because people feel like doing what they want to do (DD), is just not worth the wait times or worth being third person down on the list that their FC might call on if 'so and so' can't raid tonight.
A 5 man group means for every 1 tank and 1 healer you've got 3 DD instead of just 2. A 6 man group like FFXI means you have your standard 1 tank, 1 healer, 3 or 4 DD, or possibly 3 DD and a support class who can offer some buffs, DD, and splash heals when needed.
I can see the argument that the 4 man groups feel a bit smaller and more personal perhaps, and I am wondering how other people feel about it?
Would it not have been a far better decision to go with a larger party size to accommodate more DD and possibly support roles knowing that in most MMO communities these roles make up the bulk of players?