I think most people on the PP forum lump all their accounts into one big PP. That way you can optimize placing assets in the most advantageous account to minimize tax exposure, transaction frequency, transaction fees, expense ratios, total number of funds, and other factors. This may in fact be the only way to build an orthodox PP using 401k plans since those plans rarely have suitable gold or long term bond options.
That said, IMO that level of optimization is at the level of fine-tuning and is not required. A separate PP in each account is probably OK.
That would be the long-term bonds, right? It goes: bonds, cash, stocks, then gold, right?
You can either look at this in a timeless way or optimize for the tax code as it stands right now.
The US seems committed to the paradigm of taxing interest and dividends yearly at a higher rate and capital gains only when realized and at a lower rate. In principle the most tax-efficient asset would be one whose returns come entirely from capital gains and the least tax-efficient would be one whose returns are entirely dividends/interest. From that perspective, ranking of PP assets from worst to best tax treatment (equiv. highest shelter priority to lowest) is: cash, bonds, stock, gold.
Right now cash interest rates are practically zero. In this environment, which is a bit of a historical oddity, the ranking becomes: bonds, stock, gold, cash. So there's an argument that you should shelter assets in that order. However you'll need to move things around again when interest rates eventually pick up.
(IMO this topic has actually been discussed extensively on the Crawling Road forum. Maybe you are searching for different terms than the ones that are used there?)