4 x 1024 x 1024 to get the actual number of bits.
divide by 8 to get bytes.
Divide by 1024 to convert to MegaBytes.
(or simplify by removing one multiply and one divide by 1024)
Result = 512 :-)
Presumably 4Gbit, so 1/8th of a GByte.
I should probably also have said; just b'cos one has a less-than-aesthetically-pleasing goo application doesn't mean they all have. I'm sure there's temperature monitoring tools that can help you see if there's any deficiency, without the need to pop the covers.
Regarding the imperfect thermal paste;
Why would this be a problem? As long as the thermal conducting surfaces are covered, simply installing the heatsink will squish out any excess while not set; leaving the required thickness and coverage to perform.
Sure; it's less pretty, but do you see it every day? And overspill onto the green varnished parts of the chip(s) isn't going to have any effect.
Better people leave well alone than cause mass panic of the less capable ripping off heatsinks, sandpapering chips, and under applying paste to a single corner, thinking this will ensure optimum performance or something...