A TRACON Controller described a loss of heavy separation when post-review equipment indicated 3.93 when four miles was required. The reporter expressed concern that determining this level of spacing is impossible on RADAR.

Narrative

I turned Air Carrier X; a heavy B747; onto the LOC behind another heavy. The preceding heavy was assigned 160 KTS. When I turned Air Carrier X on [course] they were assigned 180 KTS. I was just starting to notice that aircraft were picking up about 30 KTS on base. I still had more than four miles and told Air Carrier X to slow to their slowest practical [speed] and asked what that speed would be; they said 140 KTS. I knew that speed would not hold up inside the marker; so I decided to vector Air Carrier X out for re-sequence. I never lost four miles; but when I arrived today at work I was told I only had 3.93 in stead of four miles. Well what is seven hundredths of a mile? I don't want to do the math; but I believe it's about 380 FT. I'm looking at a range of about 75 miles on my scope; so I say it is impossible to judge 380 FT. Also; on my RADAR scope when I use the slew ball measuring technique; every time I try to measure five miles I get either 4.93 or 5.06 miles. So; if Management's equipment is so accurate; how come I'm not using it? Or is their equipment that accurate? When was the last time it was calibrated? Who calibrated it? Give me a break; I should be getting a pat on the back for recognizing that the four miles wasn't going to hold up and re-sequencing Air Carrier X; which caused me to spin three aircraft on my downwind. [I] recommend using excessive spacing just in case.