I could see those 4 legions put to good use in an expanded Dacian conquest. Or in potentially holding Mesopotamia.
Mesopotamia is probably the most obvious option for any emperor. It is rich with high population. Some of them potential firendly greek cities. Latest when the trouble starts during Neros reign more legions could lead to a more ambitious campaign. Imagine Corbulo gets 40.000 more men.
Another option is going going back to the Elbe border. And from there support a dacian campaign. Which makes a nice new border.
During Claudius times, the Chauci were almost conquered by Chaucicus and again later by Corbulo, after their fleets raided the gallic coasts. But Claudius called them back.
The Chatti were almost conquered by Galba, following Caligulas campaign in 41. But it seems Claudius was not willing to keep the territory.
Around 50 AD, Vennius the roman client king of the Marcomanns was attacked by his nephews. The Lugii, the Quadi and the Yazyges were involved. A perfect casus belli, but Claudius did nothing. Like Tiberius did nothing to help Marobodus in the same situation a few decades earlier.
The Bructeri living north of the Lippe and near the Rhine were anihilated by their neighbours. Again the romans did nothing.
And the Cherusci have been in the mid of a civil war, which led to their desintegration a few decades later.
Claudius was busy in Britannia. But without this useless adventure and 40,000 men more at the right place ...