Available RAM less than what my Macbook is supposed to have When I go into Photoshop and then I look at performance in the preferences, it says that I have "3240 MB Available", whereas I have a 4gb RAM MacBook Pro.
Also, it often crashes, and I frequently have to restart it. (minimum once a day)

Can I be sure it's a hardware problem ? 
And if not, do I have ways to test it ?
If you're using a Mac with an integrated graphics card (all 13 inch models) then some of the RAM is used for this and obviously some for the OS, so this seems normal. As a rule of thumb you wouldn't want to give Photoshop more than 70/75% of what is remaining. If you assign too much to it the system won't have anything left for any other processes and we have to do a lot of work creating swap files to move temporary files about. This could be a cause of crashing. On the subject of crashing, if you're copy of Photoshop is not official (not suggesting it is) and not updated then its to be expected.