A must have for the fans of the series, specially if Metal Slug 3 left you wanting more, however...
For example, I don't know if the they forgot to correct it , but the fact of having unlimited credits greatly reduces gameplay and difficulty. Metal Slug 3, however, forced you to begin the mission again once you had lost all your lives, and you didn't have unlimited continues. This...problem in the game's difficulty results in finishing the game in 15-20 minutes in HARD difficulty!!! This, of course, makes the game boring after finishing it 3 or 4 times.
The level design is also poor, since the missions are rather short and don't have as much different routes as Metal Slug 3; this messes up the replay value significantly.
Other detail in the gameplay is the lack of consideration of the player's...efforts. I mean, they should give you some rewards for getting all the treasures, or for rescuing the hostage soldiers, since it's quite difficult to complete a mission without dying.
However, the game also has it's good things. The soundtrack is amazing, specially in Metal Slug 5, where they put some Heavy Metal in it. Most of the boss battles are interesting and challenging. The graphics are very nice too, and got a significant boost in Metal Slug 5.
Conclusion: If you're a fan of the series and Metal Slug 3 left you wanting more, you should get it, if not, DON'T buy it, you'll get bored of it in 4 or 5 days unless you have someone to play coop with.