We report on two programs to address differential evolution between the radio-loud and radio-quiet quasar populations at high ( z > 4 ) redshift .
Both programs entail studying the radio properties of optically-selected quasars .
First , we have observed 32 optically-selected , high-redshift ( z > 4 ) quasars with the VLA at 6 cm ( 5 GHz ) .
These sources comprise a statistically complete and well-understood sample .
We detect four quasars above our 3 \sigma limit of \approx 0.15 mJy , which is sufficiently sensitive to detect all radio-loud quasars at the probed redshift range .
Second , we have correlated 134 z > 4 quasars , comprising all such sources that we are aware of as of mid-1999 , with FIRST and NVSS .
These two recent 1.4 GHz VLA sky surveys reach 3 \sigma limits of approximately 0.6 mJy and 1.4 mJy respectively .
We identify a total of 15 z > 4 quasars , of which six were not previously known to be radio-loud .
The depth of these surveys does not reach the radio-loud/radio-quiet demarcation luminosity density ( L _ { 1.4 ~ { } GHz } = 10 ^ { 32.5 } ~ { } h _ { 50 } ^ { -2 } { ergs s ^ { -1 } Hz ^ { -1 } } ) at the redshift range considered ; this correlation therefore only provides a lower limit to the radio-loud fraction of quasars at high-redshift .
The two programs together identify eight new radio-loud quasars at z > 4 , a significant increase over the seven currently in the published literature .
We find no evidence for radio-loud fraction depending on optical luminosity for -25 > M _ { B } > -28 at z \simeq 2 , or for -26 > M _ { B } > -28 at z > 4 .
Our results also show no evolution in the radio-loud fraction between z \simeq 2 and z > 4 ( -26 > M _ { B } > -28 ) .