It can be concluded that North Korea will conduct its fourth nuclear test in 2014 and probably from late April 2014 - based on data provided by Johns Hopkins University's US-Korea Institute. The US has a wide range of intelligence platforms to predict the (virtually exact) timing of the fourth test - including satellites (of course) and RQ-170 Sentinel stealth drones deployed in the region.
Agency France Press (AFP) via Australia's 7 News, reported, February 14, 2014 http://au.news.yahoo.com/world/a/21464288/excavation-work-at-n-korea-nuclear-test-site-us-think-tank-claims/ :
"Excavation work at N.Korea nuclear test site, US think-tank claims
Seoul (AFP) - Recent satellite imagery suggests North Korea has stepped up excavation work at its main nuclear test site, but there are no signs of an imminent test, a US think-tank said on Friday.