Now that the rulebooks have been published, we have a much clearer answer to this question.
From Chapter 9: Combat, under Surprise:
Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.
If you’re surprised, you can’t move or take an action on your first turn of the combat.
Thus we can see that losing your first turn or not has nothing to do with your own Dexterity (Stealth) result, and everything to do with your Wisdom (Perception) result.
So, in your first example, with 4 non-stealthy goblins (passive Perception 13) vs. a stealthy Rogue and a non-stealthy Fighter and Wizard: The goblins detect the Fighter and Wizard, and the party detects the goblins. Since everyone detected at least one threat, no one is surprised, and no one loses a turn.
In your second example, where two goblins rolled high for Stealth and two rolled low (N.B. the DMG advises one roll for the whole group of identical monsters, but of course, if the DM chooses to make separate rolls, it's their prerogative): the PCs notice the poorly-rolling goblins, so they are not surprised, and do not lose a turn.
An additional point to consider:
There are two alternative methods for handling the party's stealth checks, but the rulebooks don't explicitly define which situations are eligible for these methods, so it's up to the DM to decide which, if either, of these can be applied for stealth checks. But since, without using them, any PC that fails to be stealthy spoils the chance for surprising the enemy, either one would be an improvement over a group of PCs in which one or more make no efforts to be stealthy. Both of these are described on page 175 of the PHB:
Sometimes two or more characters team up to attempt a task. The character who’s leading the effort—or the one with the highest ability modifier—can make an ability check with advantage, reflecting the help provided by the other characters. [...]a character can help only when two or more individuals working together would actually be productive.
When a number of individuals are trying to accomplish something as a group, the DM might ask for a group ability check. In such a situation, the characters who are skilled at a particular task help cover those who aren't. To make a group ability check, everyone in the group makes the ability check. If at least half the group succeeds, the whole group succeeds. Otherwise, the group fails. Group checks don’t come up very often, and they’re most useful when all the characters succeed or fail as a group.
In lieu of either of those options, a party that wishes to avoid ambushes should at least do one of these two things:
Have the stealthy Rogue scout a bit ahead of the Fighter and Wizard.
Have the Fighter and Wizard make stealth checks, even if they do have poor modifiers.
By staying close to the Rogue and making no attempt whatsoever to be quiet, the Fighter and Wizard completely nullify any such efforts on the Rogue's part.