At that level, a dragon is still going to make most of their saves. If you put four CR 12 dragons against four 16th level characters, you will get an "average" encounter. four CR 15 dragons will be a challenging encounter. An adult red dragon is CR 15. It has For +18, Ref +13, Will +17 and SR 21. Assuming they don't heroism on themselves or whatever, it can make DC 27 over half the time for Fort, about a fourth of the time for reflexes, and half the time for Will. A fourth of the time, any given spell will fail to beat their SR. If the four PCs open up with game-ending spells, generally two of them will be out of action after a round. At that point, the party can begin to explore what it's like grappling with a dragon or being breathed on.
That's pretty formidable... especially considering we haven't permitted the dragons to select feats or cast spells.
Dragons were picked to be tough for their CR in 3.5E and a CR+3 multi-opponent dragon encounter is about as nasty as it gets. A party in such a situation will lose members if they do not use every resource.
My only comment is who is doing the best in this scenario:
1) The Fighter trying to full attack with a bow?
Assume 27 Str, a strength +1 holy bow and an 18 dex with +5 arrows (possibly provided by the cleric using greater magic weapon). He does 1d8 + 5 (magic) + 8 (strength) +2d6 (holy) per arrow with attacks of +25/20/15/10 (hitting on a 4, a 9, a 14 and a 19). That';s 1.9 hits per round (assume 2 so we don't worry about crits) for about 50 points per round. He can drop a dragon in about 5 rounds (253 hit points, we'll round up).
If there are range increments invovled then this is not good and he'll be soaking 14d10 breath weapons while sniping.
2) The cleric firing off a destruction spell as a readied action as the dragons close (likely DC around 27) with a 95% chance to hit AC 8, a 75% chance to beat SR (assuming no magic item or feat that helps) and a 40% chance to kill it outright. If it saves it averages 35 damage which isn't bad. That's hititng the tough save for a CR equivalent creature and is a reasonable spell for an 16th level cleric to have one or two copies of in core.
3) We'll skip the rogue -- this type of encounter does not play to his strengths (flanking under these conditions is brutal).
4) The wizard might have an ideal 8th level spell: maze looks promising if the party is lucky.
Low level spells (that could be prepared) are also interesting: Resiliant sphere might actually take a dragon out until their allies are defeated. Even at DC 24 (for a lower level spell) it has a 75% chance to break SR and a 50% chance to take a dragon out of the fight for 160 rounds. It's close range but still better than what the fighter can do (range increment for a composite longbow is 110 feet), range for this spell is 65 feet.
Notice that this is a 4th level spell targeting a weak save (the classic wizard trick).
Hold Monster has a range of 260 feet, assume DC 25 so a 75% chance to break SR and a 35% chance to drop a dragon from a great height.
Now in core, with bonus feats, a wizard (having limited feat choices) is likely to have spell penetration. No wizard will have all of these spells prepared . . . but they are likely to have some of them (the low level save or suck that targets reflex is an especially common choice) and all of them are better actions than the fighter's attacks at stopping a dragon.
Or even compare to damage spells. Hit all 4 dragons with horrid wilting (requires them to be grouped). 1 will resist. 2 will save for 28 hp of damage. 1 will fail and take 56 hit points of damage. That's an 8th level spell gone -- whereas repulsion (save or suck) will keep half the dragons out of breth weapon range (a lower level spell with a much more effective outcome).