1. pwede ba lagyan ng rear view camera ang G4 and san pwede ipalagay and ano brand ang maganda at a reasonable price.
yes. sa bumper, drill-hole type. or sa license plate, bolt-on type.

i use a China-made camera. more than a year na and still working. costs P1,500 in Banawe with 1 year warranty. connected to Pioneer AVH-1450DVD head unit.

if you want branded cameras, you can buy Pioneer, Sony, Kenwood, Panasonic, etc. these costs more, from P5,000 and up.

if ever ipa install sa labas will it void yung casa warranty?
warranty will only be voided if problem/damage is caused by the added part. para sure ka, call the CASA before you install anything.

yung monitor ba nito will be another/separate screen or ico-connect nalang sa existing screen sa me dashboard?
meron model na may kasamang separate screen. meron model na camera lang. if may AV INPUT ang stock screen mo, then pwedeng camera lang ang bilhin mo.