The original debris map of Reactor 4's Spent Fuel Pool (created by Hitachi) was released by TEPCO on April 23, 2012 in the document for the Working Council for "Mid-and-long-Term Roadmap towards the Decommissioning of Fukushima Daiichi Nuclear Power Units 1-4". The US Nuclear Regulatory Commission has the decay heat map of the same pool created by the US Department of Energy ("Official Transcript of Proceedings", released on May 26, 2011 (link is PDF), page 188). You overlay these two maps, and you'll get this:
The Reactor 4 SFP contains 204 new fuel assemblies and 1331 spent fuel assemblies.
(H/T anon supporter of this blog for the map. H/T reader Atomfritz for the NRC link. Thank you.)