Me and my husband will the traveling to Chile in the end of March for 3-4 weeks.
We both love to hike and trek, were originally looking into a trip including Willarica traverse.
After some reading it looks like it may be too late in the season for this hike.

I am at loss and would like to ask you for a help and advise.

What hikes and treks (multiday) would you recommend for April so that we can piece them together and make a hiking vacation. We will fly to Santiago and will be relying on a public transportation.
Are there any areas for trekking in Chile / Argentina which have stable good weather? Since we have limited time, it would be shame to choose a trek in an area where it rains a lot and not being able to enjoy the scenery.
Thank you for your help.

late march / early april should be fine for just about anywhere in chile/argentina... let´s say below 5000m

My suggestion is trekking the 'W' or the 'Circuit' at Torres Del Paine National Park. I did the following 3 years ago and it was a great experience. From Santiago book an early morning flight to Punta Arenas (aerolineas argentina has flights). Then take the afternoon bus from Punta Arenas to Puerto Natales. Get into Natales in the evening, find a hotel, buy supplies and get a bus ticket to the park. Next morning take the bus to the park and you will be hiking by noon. So even though the park is a good distance from Santiago, it only takes a day of travel to get there, and it is easy travel. The park is completely worth the travel. The 'W' will take 4 days or so, the circuit will be 8+ days. The park is amazing and completely worth the trip.

http://www.torresdelpaine.com/ingles/secciones/02/a/mapas.asp#