Encounters per day - in 4e this can be any number up to the healing surge depletion limit. A single moderate difficulty fight is viable if the players don't know it's the only fight. In fact it's ok for the number of fights per day to be random. The main reason to limit # encounters is the massive time sink; 8 moderate-difficulty fights in a day could be 16 hours of play, for me that's probably 4-6 sessions, 2-3 months of fortnightly play! The published WotC adventures are written like that and work very poorly.
Encounter difficulty - in 4e this can be anything from a massive 'Spike' battle that tests PCs to the limit (from which they may need to flee, and possible PC deaths) to a moderate battle maybe EL 1 under Party Level.
But that is still really a fairly limited range, say EL-1 to EL+7 at higher level, with EL+7 about 4 times the difficulty of EL-1.
What 4e can't do well is the really trivial fight, it will take ages, be dull, and not attrite any resources.
The kind of videogame-based "fake megadungeon" approach Angry DM is taking, with mini-bosses at the end of each 'day' sequence, looks like an appropriate approach for 4e. But in 4e there is really no meaningful Exploration element, the game doesn't support that at all, and IME is even best done without a map - just telling the PCs "you trek through the megadungeon, until... (Encounter X)" is what works best in that system. 5e is completely different and IME (GM'd 34 sessions of online Wilderlands 5e sandboxing) the design supports old-school exploratory play. I use mostly OSR material (and Caverns of Thracia) in my 5e game, and it works a charm. I found using Dyson's Delve dungeon maps a complete waste in 4e, but in 5e they work a treat.