When should I request clearance to return to altitude, after leaving cruise altitude?
The “cruise” terminology you are questioning is really not used anymore. It still appears in manuals, but it was very confusing and fallen out of use. The intention of a “cruise” clearance was to allow the pilot to climb or descend as required with the specific cruise altitude window. In your question that was 4000’-6000’. However, today people the same request would be made as “request an altitude block 4000-6000”. ATC would respond “(callsign) is cleared the block 4000 to 6000.” No further calls are required from you while maneuvering in that altitude block.
Sometimes a “cruise climb” clearance will be issued to heavy aircraft to allow them to climb slowly and level off for a while when they are too heavy to climb all the way to their final altitude. In that case they can climb as they are able but are not allowed to descend out of an altitude they have already climbed to.