Ambient 3: Days of Radiance is actually an Eno-produced album that features the music of Laraaji. He plays instruments like the amplified zither and the hammered dulcimer. Very tribal. Very cool. Ambient feel even though it's not electronic.
Ambient 4: On Land is the last of the series. Wikipedia quotes Eno explaining: "the making of records such as On Land involved feeding unheard tape into the mix, constant feeding and remixing, subtracting and 'composting'".
They're just great albums! :)