Metroid 4 is quite a peculiar game in the Metroid series. It is much more linear, much shorter than some of the other offerings from the series. However, it is still fun. This is Samus' first game for the Game Boy Advance.
The graphics are quite nice for the GBA, though you may want a wormlight. Controls are a bit awkward at first, but the learning curve is quite short and you'll be running, shooting, and morph balling around in no time. There are quite an array of new weapons and powerups for Samus to earn, as well as the venerable Energy and Missle Tanks. The music is solid, but not extraordinary. Sound FX are well done.
This game is quite short (I can't forsee any average player taking longer than 5 or 6 hours to complete it) so I wouldn't recommend paying full price for it. However, the gameplay remains true to the Metroid series. Gameplay is what makes this game worth the money (especially if you can get it used). If you can find it used, Metroid Fusion will not disappoint and you'll welcome it as a valued game in your collection.