lowering blocks

got a 91 mazda b2200 5spd carb. short bed standard cab.

curious as to how low of lowering blocks i can go? i was thinkin 5"(lowest i can find) but im not sure if this will mess up the driveshaft length or something goofy like that.

i want at least 4" cuz ive seen lots of 3" drops and it looks to high still. so is a 4" or 5" drop 'safe'. im sure ill also need shocks/struts or whatever, i think they are worn out now as it is.

thanks for any help!