in Travian 4, which nation has the best Defence hammer in each time line of game, include early game, middle age and End game Time?
Hm,there is not much talk abut defense hammers around since sending defense from multiple villages is almost as efficient as hammer,not to mention easier to feed(no crop/oases needed).
The obvious choice are Gauls since they are the defensive tribe.
In early game phalanaxes are not too overpowered but they are fairly cheap and in addition with the wall can defend you...getting druid riders early enough is devastating to the attackers in early game since most of enemy armies consist of infantry...by the early mid game(before you get haeduans) you will be easy target for attackers with cavalry(since getting haeduans is somewhat longer for gauls then other tribes),but once you get them you are back on the defensive throne.
Praetorians and Druids were two most used defensive hammers in T2, T3 and T3.5, so they might just be the same in T4.
But as Arremer pointed out, defensive hammers are quite bad for the number of reasons (time to build one, time to rebuild, crop consumption, highest starvation priority, vulnerability to city nuking), so having multiple villages with (relatively) medium sized packs of cheap infantry phalanxes/spears/praetors is much better.