Measurements of the temperature dependence of the resistivity {rho}{sub {ital a}},{rho}{sub {ital b}}, resistance {ital R}{sub {ital c}}, and thermoelectric power {alpha}{sub {ital a}},{alpha}{sub {ital b}},{alpha}{sub {ital c}} for a YBa{sub 2}Cu{sub 4}O{sub 8} crystal under high pressure verify that the CuO{sub 2} sheets are oxidized by pressure-induced electron transfer from the CuO{sub 2} sheets to the Cu(1)-O double chains. The anomaly in the CuO{sub 2}-sheet resistivity {rho}{sub {ital a}} at {ital T}{approx_equal}170 K, which moves to a higher temperature on oxidation of the CuO{sub 2} sheets, is correlated to a change in the character of {ital R}{sub {ital c}}. Our data also demonstrate unambiguously the contribution to the transport properties from Cu-O chains. {copyright} {ital 1996 The American Physical Society.}