I suspect theory and practice may be two different things in this case, since we distinctly encountered humanoid groups of this size in the 5e introductory adventure (the well-respected "Mines of Phandelver"). They were tough but not deadly, and I believe that reflected the intent of the game design which was to raise the stakes of combat back to an old school level. So I don't believe you should be applying this level of mathematical rigor to your conversions. 4 Hobgoblins is 4 Hobgoblins in pretty much every edition, just the stakes are different (and therefor the players' approach to combat needs to be modified from edition to edition).
Possibly mathematical divergence might need to apply at higher populations (the old 0e random encounter with 600 Orcs)...
Where I would agree caution is advised is when monsters got rules overhauls (particularly demons and devils spring immediately to mind. The 3.5e Barbed Devil is a motherfucker, for example).
One of the difficulties is guessing to what extent the designers have their thumb on the scale when it comes to encounter design; does "deadly" actually mean "chance of a character dying if the party is both foolish and unlucky"? Is an appropriate challenge for a given level really that challenging? Is an appropriate challenge for four 3e level 1 characters
really one 3e level 1 character?
This makes anecdotal evidence tough to assess. When encounter design is so skewed to players never facing any real risk, beating four hobgoblins in both 3e and 5e doesn't really mean that hobgoblins are equivalent in both editions.
So I work with two assumptions. The first is that late-edition designers have their thumb on the scale to the same extent in 3e as in 5e. The second is that designers have more or less succeeded in creating an encounter design system that reflects that framework.
Going to the assertion that 5e is supposed to be more dangerous, I think to the extent that is true, it was accomplished by making the monsters tougher. I remember when Phandelver first came out, seeing some commentary that 4 goblins gave players more trouble than they expected. Well my model, based on what the designers say about their own systems, predicts that goblins are a notch tougher in 5e, and orcs/hobs are also a bit tougher.
Something similar was done with 4e, but in the other direction. 4e assumes that fights against multiple creatures are more interesting than fights against one or two. As a result, they dramatically reduced the power levels of a number of creatures, so you could have more of them in a fight. So in that system, you can't assume that a hobgoblin is the same as a hobgoblin in any other system, replacing them on a one-for-one basis would make all the encounters entirely trivial.
So my assumption is, if the designers' encounter design rules suggest that a party of a given level should be facing a greater or lesser number of hobgoblins than it would in another edition, it is a deliberate design objective.
And really, I need something if I am going to adapt material quickly. And I need a way to assess creatures that are unique to 5e, which includes a host of NPCs. And I need a way to spot when WotC has decided to change the power level of a monster; some demons and devils have varied wildly in their power levels between editions.
This is the crux, Beoric - what I've been trying to say, but 1True puts it so much more succinctly.
DM-fixation on balance is something that many players claim "sucks the soul" from the game, because the DM's goal is not to make a perfectly balanced adventure (which is essentially analogous to railroading your party's survival, ensuring they'll never meet actual dangerous threats so the game can advance at a preset pace). The DM's goal is simulation of the world and arbitration of the rules. The solution to an overpowered encounter is not on the DM's shoulders if the players are given free reign to approach or disengage at will, as all players ought to be given. The OG random encounter tables had "Red Dragon" and "3d100 Orcs" on them because it was expected that the party would pick and choose their battles when outmatched by stronger foes. Fighting 200 orcs as a party of level 2 characters isn't balanced, but those kinds of encounters are still found in some of the best, most critically-acclaimed adventures.
It has nothing to do with balance. I don't need encounters to be balanced. But I do want to know what I am creating, and I want adaptations to give an experience as similar as possible to the original. And I want to be able to telegraph to the players what they are facing, other than "There's a troll - oh, too bad, so sad, this troll turned out to be more dangerous that you thought it would be, based on your encounters with other trolls." .Plus, knowing the relative toughness of creatures has an impact on how the creatures relate to the world, and their position in it.
Anyway, I have what I need, so I'm dropping this topic now.