As a veteran gold box addict, let me explain why 6-8 party members is a must have. My PoR party always had: a pally, fighter, fighter-cleric, cleric, cleric-mage, and a thief-mage. Note these are multi-class not dual-class characters.
With only 4 characters limited to pure classes, you don't have a front line. You have "the better stat" unit killing the weaker unit over and over ad nauseum. Now, compare this to how I use my party like a discreet team of specialists.
My pally/fighter are the tanks: huge HP and good melee. The f-c and cleric back them up with decent melee, and they have enough defense to heal any front liner who takes too much damage. (At some point, the tanks eventually get hit from behind.) This wall separates enemy swarms from my mages, who are both equipped with sleep and stinking cloud. The melee units guard the mages long enough for them to disable most enemy units, who are then instantly slain. This is basic crowd control, and any good team should be able to do it.
Take one typical situation where one group of mobs is dead ahead while a stronger (even smaller) group is flanking your side. You can equally distribute your tanks to each side with backup WITHOUT worrying about a creep slipping by. In a pincer attack, you scramble your team to cover the mages.
My thief-mage switches to a bow when needed: when out of spells, to kill helpless units, and to counter enemy casters. Those thief levels add good HP and attack rolls to an otherwise useless unit.
With a main healer and two backup healers, you have plenty of resources to adventure with. However, you must manage them. I make sure not to deplete any single cleric of his/her heals while adventuring. Undead get rocked by the pure cleric's more potent Turn, and cleric lvl 2 Hold Person adds even more to the team's ability to disable-then-slaughter enemies. The Hold spell is also vital to stopping charmed party members.
With a "standard party" of 4, you're very limited in combat options. You can't block off opponents from reaching your only unit capable of crowd control. Your team therefore takes more damage. You're dependent on a single (overworked) healer. You're basically forced to choose the Fighter/Thief/Mage/Cleric team in order to overcome all the game world's obstacles.
With a party of 6, you get to develop a team based on the *idea* of the standard party. You have room to fit the 4 core skills into a group and even stock abilities you want. You can further diversify characters with feats and equipment. I strongly recommend increasing the party size to at least 6.