Hi! Up on the glacier it's sort of early spring - crevasses covered, can be quite wintery at times, but warm and nice if sunny. The climbing routes are going to be in winter condition - that is, covered by snow, loose stones frosen, cornices. Approach to Kebnekaise fjallstation is wet and more ofter than not can not be done on skis,Tarfala valley and above are skiable at least until mid-May, often until early June. Nygren's this time of the year is a rather simple mixed AD. Start early, it faces east and the snow gets soft pretty fast. Use the eastern standard route as descent, it'll be snow-covered and thus very easy to descent.
You can book transport to Kebnekaise Fjällstation if you arrive late, for instance. That's a 19 km trip. Hike from there. In April it's still winter in the Arctic. The weather can be rather rough. Daytime can also be sunny and warm, if you're lucky. Depends a lot on of it's early or late April. http://www.nikkaluokta.com/index.php?id=1522