Ambleside is cracking eMTB territory. Four hours out and back gives you a decent radius into the Lake District fells, and the trails hold up well for a full-power ride.
A solid loop from Ambleside would take you up towards Loughrigg Fell or swing south to Grizedale Forest, which has marked trail centre stuff alongside more exploratory riding. If you want something wilder, head north past Rydal Water and up onto the high ground around High Street, though that gets committing quickly. The climb out of Ambleside on any route is going to ask something of you early on, so worth thinking about your battery strategy before you set off.
On range: four hours in the Lakes means climbing. Plenty of it. Running a mix of eMTB and Tour+ modes rather than sitting in Boost the whole time should see you comfortably home on most 700Wh+ batteries, but if you're on something smaller or you're going hard, eco on the road sections will buy you headroom for the technical stuff.
Worth checking trail access before you go, particularly on the higher bridleways. The Lakes has a decent network of legal routes but it pays to cross-reference with the Harvey Maps or the Cumbria MTB guides rather than assuming everything marked on OS is rideable.
What battery are you running and which bike? Can tighten up the range estimate from there.