This quote apparently comes from a 1916 USDA study on Hemp Hurds as a Paper-Making Material,

The most important point derived from this calculation is in regard to areas required for a sustained supply, which are in the ratio of 4 to 1. Every tract of 10,000 acres which is devoted to hemp raising year by year is equivalent to a sustained pulp-producing capacity of 40,500 acres of average pulp-wood lands. In other words, in order to secure additional raw material for the production of 25 tons of fiber per day there exists the possibility of utilizing the agricultural waste already produced on 10,000 acres of hemp lands instead of securing, holding, reforesting, and protecting 40,500 acres of pulp-wood land

Thus, this quote would have pre-dated modern manufacturing techniques for paper production as well as modern tree-farming techniques. This means that this is actually a difficult question to answer in part because tree farms and traditional farming techniques are completely different from each other and modern technology means that the techniques involved have likely changed greatly in the almost 100 years since the study was conducted.

To begin looking at things from a modern standpoint, tree farms typically thin a stand at 15 and 24 years with a final clear-cut harvest every 33 years (see slide 5 of 15). However, hemp can be harvested on an annual basis which puts it in line with traditional farm techniques1. This would imply that on a very simplistic level the hemp could come out ahead just because no thinning or harvest of a tree stand was done in a given year.

So to give a bit more of a fair comparison we are going to need to look at aggregate data and and look at the average yield per year. As with traditional farming different stands can result in different yields and in general harvesting sawlogs is more desirable than pulpwood (which is used to produce paper) since sawlogs are more valuable and therefore more profitable. This in turn means that stands tend to be managed with maximizing the amount of sawlogs produced.

The most commonly farmed trees for pulpwood appear to be Loblolly Pine, Acacia, and Eucalyptus with Eucalyptus being the most common. Based on my research, Loblolly Pine and Eucalyptus productions seem to be as follows:

The yields for hemp fiber3 on the other hand tend to be based more on historical data and tend to place the yields at 2 to 12.5 tons/acre/year with 5 tons/acre/year being the average on a good year. A modern report from a Kentucky farm put the yields at 2.8 to 6.1 metric tons/acre/year with other sites getting worse yields.

Thus, to return to the original claim, if we assume that only paper production is considered (i.e. fiber production in the case of hemp and wood pulp production in the case of trees) then the claim appears to be false with hemp yields only appearing comparable to Loblolly Pine per acre/year wood pulp yields although it is possible for hemp to out produce Loblolly Pine production when it is not grown for pulpwood.

However, if other trees are considered then well managed tree farms appear to greatly outproduce hemp farms in terms of per acre yields. The only way this claim is true is only if year-by-year data is examined due to the way tree farms are thinned and harvested due to the significant number of years of no production.