Land has strength of versatility since it can't be bogged down to one terrain type or the sea. It's weakness would be small size and relative lack of armor.
Air has speed as its strength. But its weakness is that it has little in the way of defenses.
Ship has strengths in firepower and armor but loses in speed, versatility and is bound to water when in cruiser form.
Space has the major disadvantage of always being accounted for. (No villain will build their lairs and bases above ground and visible if there's a hero-side Kill Sat that can nuke the place.) Also, it's a large and visible target with minimal if any evasion capabilities. On the flipside however it has the greatest attack range of anything and the greatest visibility on things.
I'm inclined to suggest that Air has the greatest versatility, given that it's presumably capable of covering all terrains that Sea and Earth can do; I'm inclined to agree, however, that it has speed as a strength and a lack of armour as a weakness. Rather than small size as a weakness (it does, after all, make one harder to hit), I'd suggest low firepower: it might carry a fighter's complement of missiles and machine gun, but is nevertheless outgunned by the other three.
Earth I suggest might have the second-strongest armour after Sea, and in terms of terrain complements Sea: where Sea does not go, Earth does, and vice versa. It may also have a travelling range advantage over Air, as it may be more fuel-efficient or be able to take advantage of more varied land-based fuel sources, while Air might be tied to landing strips and jet fuel (if it uses jet fuel). I also imagine Earth as carrying the third-strongest weapons: cannons of power similar to Air's missiles but having greater stores of ammunition, and comparable machine guns. In terms of weaknesses, Earth seems likely to be fairly slow.
I agree with Major Tom on Sea and Space.
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What?
Hmm, perhaps Earth is not exactly sluggish but isn't capable of doing hairpin turns at high speeds. That or he just doesn't care about obstacles and would plow through nearly anything.
Oh BTW these robots are composed of materials not of human origin.
edited 1st Dec '12 2:17:02 PM by Worlder
On further thought, if the robots are all of similar size then I'd likely imagine Earth to have the strongest armour, and perhaps follow a schema something like this:
Earth I associate with stability, strength (especially defensive), but also with slowness.
Air I associate with speed, a degree of remove from the situation and accuracy, but also with defensive weakness.
Water I associate with travel (it may act as a transport for Earth and Air), manoeuvrability, and with a degree of strength, perhaps especially offensive; a water-aligned robot might be limited to remaining in water, however.
Space I associate with great remove, both in time and space, with great offensive strength, and with defensive weakness: it might have low armour and very powerful attacks, be difficult to reach but relatively easy to destroy once reached, and be limited to using its most powerful weapons only after significant charging time, or have those weapons output power relative to charging time.
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Um, actually they are the size of their respective vehicles.
The ship is going to be a colossus relative to the others.
Fair enough; in that case my comments on Earth and Water revert to those in my original post, more or less, I believe.
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There are four robots. One turns into a tank, one into a jet, one into a cruiser-carrier naval ship, and the one that turns into a Kill Sat.
Land, Air, Sea, and Space.
I need some strengths and weakness for the four.