In HGSS, what Legends do respawn after beating E4?
Correct me if I've made any mistakes, but I'm pretty sure the complete list is:
Entei, Raikou, Suicune*, Ho-oh, Lugia, Articuno, Zapdos, Moltres, Mewtwo, Latios and Latias (this applies both to the roamer in your game and to the upcoming Enigma Stone event that makes the other of the pair available), Groudon, Kyogre (version-specific), Rayquaza, the red Gyarados at Lake of Rage, the Sudowoodo blocking Route 36, the Snorlax blocking Diglett's Cave**, and the Lapras in Union Cave every Friday***.
In short, basically every legendary and stationary Pokemon.
* If you defeat Suicune at Cerulean Cape, it will respawn in the Burned Tower after defeating the Elite Four again.
**I'm not COMPLETELY sure about this, as I just went ahead and caught it the first chance I had. Logic would dictate that it would respawn too, however, since pretty much every stationary overworld Pokemon + legendaries do as well.
***Lapras will respawn every Friday regardless of whether you capture or defeat it.