If I go to the US for 90 days on an ESTA, then go back to Norway for say 3-4 weeks, can I then go back to the US for another 90 days?
Unlike staying for more than 90 days at once, it's not explicitly forbidden. In the US, you are allowed a certain length of stay per entry, not X days in a given period like in many other countries. And going to Norway would end your first stay, unlike a short trip to Canada. So as long as your ESTA is still valid, you could try to enter the country.
That said, you might get some questions and should better have a good explanation for your trips. If you use up the 90 days once more, you will have stayed six months almost without interruption in the US and entering a third time could be difficult. There is no “hard” threshold (at least not in the published rules, as far as I know) but the visa waiver program is intended for short stays and visits, not to take up residence in the US. Staying 90 days (or almost 90 days) and then returning shortly after leaving could suggest you are trying to circumvent the rules.