Ram and heatsink!!

Hey everyone i purchased 4 gigs of ram recently and i have noticed that when i pull up activity monitor it, under the pie graph it only point out 3.75 GB. But free = 1.87 and used =2.13 gb so it adds up, right?

My first question is why doesnt it say 4 GB under the pie graph. And my next question is ever since i downloaded iStat for some reason Heatsink A displays no temperature, but Heatsink B displays 48 Celcius. Is there something wrong with my computer??

Well only 3.75 gb because of 256 mb of ram for the video chip/card

And the heatsink , I'm I don't see a heatsink a and b on mine. on my iStat I have

HD: 27*c
CPU 52*c
CPU heatsink 41*c
enclosure base 25*c
enclosure base 2 25*c
enclosure base 3 26*c
enclosure base 4 25*c
GPU 47*c

For some reason my iStat shows Heatsink A and B after Enclosure,2,3,4.