4. There is an experimental field for testing new varieties of rice, as shown in the figure, with a total area of 40 mu. One part is planted with the new variety, and the other part with the old variety (the planting areas are not necessarily equal) to facilitate comparison of results. The old variety yields 500 kilograms per mu; of the new varieties, $75 \%$ are unsuccessful, yielding only 400 kilograms per mu, while the other $25 \%$ are successful, yielding 800 kilograms per mu. Therefore, the total rice production from this experimental field is $\qquad$ kilograms.
【Answer】Solution: According to the analysis, let the new variety be $X$ mu, with a yield of $75\% \times 400 X + 25\% \times 800$ kilograms, then the old variety is $40-X$ mu, with a yield of $(40-X) \times 500$ kilograms,

Total rice production in the experimental field = new variety yield + old variety yield $=75\% \times 400 X + 25\% \times 800 + (40-X) \times 500$ $=20000$ (kilograms).

Therefore, the answer is: 20000.