A 4-poster flatpack can be made at a workbench (Steel framed bench or better) in the workshop of a player-owned house. When making the normal 4-poster, this requires 53 Construction, uses 3 mahogany planks, 2 bolts of cloth, and gives 450 experience. The flatpack can then be used on a bed hotspot in a Bedroom, either in building mode or not. This requires no Construction level and gives no experience.
The cost of making and selling a 4-poster flatpack from scratch gives a profit of -6,010 coins.
After completing the hard Fremennik achievements, they can also be traded to Advisor Ghrim in exchange for 653 coins being added to your kingdom's coffers in Miscellania. See Flatpack: Miscellania reselling for details.