It won't make a huge difference, like I think you're saying. If the amp is properly working, I doubt you'd notice a difference swapping an 8Ω speaker for a 4Ω one. It might be slightly less loud, but probably just barely. The tonal characteristics might change slightly too, like maybe sounding a little more detailed or having a tad more bass. But again, not a huge difference.Check your tubes; You may have to take it into the shop.