Yes, generally it's more efficient to add members to the team more gradually and not all at once. This is mostly because it's harder to keep a large team at a reasonable level in the early game than it is later on, and because you don't need a bunch of team members early on. More specifically:
- The type variety and coverage provided by a full team of six is generally not required in the early game (you'll be seeing a lot of the same types of Pokemon for a while), so at this point, having extra team members might not do anything for you that you couldn't already. The extra Pokemon will only demand experience points and training from you.
- Chances are that the Pokemon you last caught is of a lower level than your current team; that means catching them up in battle and potentially grinding for experience points. Adding a bunch of new Pokemon at once means you'll have to catch them all up at once, which is a pain and obviously better avoided. This is harder and more repetitive to do at earlier stages when your training options are limited and you don't have access to the Exp. Share or Lucky Egg.
- Having a bunch of Pokemon early in the game sets you up for getting under-levelled. Even if the new Pokemon you add early in the game are reasonably strong in terms of level, raising them will detract from the amount of experience points your other Pokemon will receive. This means that your team will be at a lower level overall and thus weaker -- though this is counteracted in part by the fact you have more Pokemon to use. You'll still be playing more defensively though, since your attacks won't hit for as much damage.
Having a full team after the fourth gym definitely sounds a bit much to me; I don't think six team members are required at that point in the game. I'd consider having a look at which team members you could perhaps catch at a later point, or which members you could replace so that you don't have to train so many Pokemon early in the game.
I'd personally look at having two or three Pokemon for the first few gyms, then adding members to the team as I approach the final stages of the game. You can choose how you go about it yourself, though.
