The 4300-ft rule (US only?) refers to simultaneous independent instrument landings. If ATC keeps the aircraft staggered (i.e. not alongside each other) ILS approaches can be flown to closer parallel runways, and of course in clear weather airliners land side-by-side on SFO's runways 28L/R, which are 750 ft center to center. The legal minimum is maybe 700 ft (?).