I'm not sure what you want to achieve here; you must be trying to do something other than simply move to new servers?
otherwise I'd personally do:
1. Move DB from server 2 to new server 4
a) copy& restore database
b) run the configuration wizard on server 1 to point to the new database
[if it fails re-run the config wizard and return to server 2]
2. move the solarwinds application from server 1 to server 3.
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if you want to make sure the new servers work then:
1. copy the database from server 2 to 4
2. install NPM on server 3 [using the 30 day licence]
3. make sure it works
4. de-licence the old server
5. re-licence the new server
Not sure what stepping through an additional poller gives you in your example; if the new server doesn't work then your old server is still up and working while you get the kinks worked out.