The M4438G has a WUXGA screen, meaning it has more physical pixels and is capable of greater resolutions.
While deciding between the 3438 and 4438, my most important conclusions were:
1) It all depends on what you plan to use the screen FOR. For instance, if you're mainly watching normal DVDs or downloaded video (like me) then the WUXGA does not offer any advantages. On the contrary: when in full-screen mode you're "stretching" the movies out even more, since standard DVD resolution is only 720x480. Stretching out video invariably results in quality loss, blurriness, etc. The 4438 only becomes useful with High Definition Television or some other video source that has far greater resolution than 720x480...as long as you don't have that, the 4438 is a waste of money.
2) Of course, what the 4438 and its 1920x1200 res. does offer is a greater workspace, i.e. you can fit more windows/applications on the screen. The drawback being that any object will look smaller and text will be less legible. It's down to personal preference: me, whenever I've put my regular monitor on something higher than 1024x768 in the past, working in high-res mode only caused headaches and annoyances, so I would always revert back in the end. Other people don't seem to mind as much, though.
3) One minor consideration: the WUXGA screen has less brightness than its WXGA+ twin (it's in the specs) but I don't think it's very noticeable.