What's too hot for a internal HDD?

[Kaiju]

Currently installed a fourth drive in my Thermaltake Shark and the 120mm fan blowin' on it seems to not be up to the job anymore.

HD4 - 64C
HD3 - 54C
HD2 - 53C
HD1 - 39C

As you can see HD1 is the lowest temp due to being at the bottom (heat rises as you know). So basically, is the HD4 too hot?

 

[Kaiju]

Yeah, they are stacked up on one another, but are spaced out as much as possible. Is there a HDD stacker I could possibly buy or a HDD cooler?

 

[Kaiju]

Installed a 120mm fan and swapped the drives about a bit and voila, everything is 40C max.

Thanks for the help guys, much appreciated. (ByteJuggler especially for the lengthy and informative reply )