The terrain is rocky on day 4, but not a lot of scrambling necessary. It's cold despite the sun and many layers are necessary. At this altitude the body never heats up enough. The vegetation takes on surreal qualities (see next photo) as the group nears their destination for their 4th night, Barranco Camp.
There are several route options to take on this day. The Western Breach has recently reopened (December 2007) and hikers would stay and camp at Lava Tower for the night before heading to Arrow Glacier. Many trekking companies however still prefer to use alternative, less dangerous routes.