Well, it all depends on the situation. Done cold, and without support ... the most you're getting is that +4.
Now, if the party just collectivelyhanded the fellow his/her/it's posterior in open, relatively-fair combat ... that's another circumstance perhaps worthy of a small bonus.
If the PCs have authority-of-law (IOW, if they really ARE cops, or the equivalent of cops) and the fellow they're questioning is involved in criminal activities, that would be another (seperate) circumstance worthy of a small bonus.
If, during the GC/BC routine, the question-ee is bound up and helpless, and under (believable) threat of imminent physical harm ... that might be yet another circumstance bonus.
If the players pull the old '20s "bright light" full-scale interrogation routine (relative sensory deprivation, swapping off asking questions and keeping the subject, persistent physical discomfort [temperature, thirst], frightening environment (lots of nearby water for a Fire-elemental type subject, a big bonfire for a Plant-type subject, etc), even the proximity of a large number of torture devices -- wether their use is intended or not) ... if the players do any or all of those, some sort of circumstance bonus would seem appropriate as well.
So let's presume they pull the GC/BC routine initially described. They've just kicked the guy's butt, along with a few of the guy's friends' butts, and now have him hogtied in a darkened tent, filled with heat and smoke from a carefully-built fire, in which a couple iron pokers (or even a branding iron, if one is available) have been set to heat up. He's strung up in a painful but not damaging position, and the GC/BC pair go at him rapid-fire, never pausing or letting up; meanwhile, in the background, the other party members discuss the finer points of torture -- it's benefits, it's methods, the moral implications of it's use (it might help to have the paladin stand watch outside). Perhaps while one or another sharpens their knife or sword, keeping a steady rythmic sound of metal-on-whetstone going throughout the interrogation. Throughout this, the subject is denied water, though plenty of cool water is p[resent -- for the PCs to drink from, with obvious sounds of refreshment.
Now, that's a nasty situation, and not what I'd call good-aligned; but not evil, either (other than to tie him up, noone ever TOUCHES the subject; the hot irons won't ever be used; the discussion about torture is, and will only ever be, purely academic).
All told, I'd give the +4 (total) for the GC/BC routine ... +2 for overall discomfort ... +2 for visible threat of harm (the hot irons) ... +2 more for audible suggestion of harm (and believe me, the powers of imagination can make even the driest of academic discussions about torture or violent coercion VERY ... persuasive ... when you're tied up and the people discussing it are decidedly unfriendly), +2 for being put (and kept) off-balance (require all actively-participating characters to make a Concentration check to keep that bit up, btw, maybe DC 10 or 15), and maybe throw a +2 bone to the PC's if they're THAT thorough in putting the whole scenario together. ^_^
Thus, total modifiers, for all elements combined, could go as high as +14.
Now, since the above is, inherently, a multi-hourprocess, I see no reason not to let the lead PC "take 10" on their Diplomacy or Intimidate skill. So, if they have even 1 rank of the skill in question, they can make that DC of 35, without breaking a sweat. Of course, I was intentionally trying to be a bit generous with modifiers, on teh assumption that this information may be fairly important to the players, and that gettign it may be fairly key for advancing the campaign's plotline.
...
But wait!
While you're being that generous with circumstance bonusses in the PC's favor, don't forget circumstance bonusses to the DC ... the subject's non-present friends aren't likely to appreciate him caving in (and won't take "but they did X to me" for an excuse). Nor his employer, nor whoever he's giving information ABOUT -- noone likes being ratted out. And they, unlike the presumably good-aligned PCs, won't stop at threatening to hurt the subject, they'll do it ... or worse!
So, assess what sort of circumstances might increase the DC, and apply the right modifiers. If there's a high-level necromancer involved in there somewhere, that (and what he could and WOULD do for revenge), I'd say that's worth at least +5, maybe even +10 or more -- picture being tortured SLOWLY to death, then, being animated as an intelligent undead, only to face a THOUSAND YEARS of continued torture ... !
IOW, you're the DM ... right? Take into account circumstances on both sides of an issue. The static DCs listed in the book are just rough estimates, to use either as starting poitns, or "good enough" numbers to use when you don't want to slow down gameplay at all.
Just remember that they're only guidelines and suggestions, not hard and fast rules. Try practicing the trick of saying "This shouldboost success, as shoudl this, and this, and this". That way, when you tell your players "You have +X in circumstance bonusses, go ahead and roll", or "the DC is +Y higher than usual, but you can roll anyway" ... if asked by (possibly incredulous) players "what gives", you will be able to rattle off a list of assorted circumstances that modify the roll and/or DC.
