Your question is asked over and over. It's so hard to predict what each year will be like.
As always, just plan on layering. Bring a light weight rain coat and an umbrella (if you like) A mix of both long and short sleeved shirts is a good idea. You can use the rain jacket as a light weight jacket and for warmth with layers under.
Might might need a jacket daytime, or maybe nighttime, always a good idea to have one...as mentioned, a rain coat can be used.
I'd suggest ONE sweater, again, lightweight, perhaps a Merino Wool (like Ibex or Smartwool, etc)
Bring a scarf, and you'll fit right in, and have that extra layer of warmth. Nothing like something around the neck to help hold heat in the body...
Suggest you look at the weather a week out of your actual trip, and plan that way. If it looks very rainy and cold, pack a few more layering pieces, if it looks warmer and drier, then not a much.
Further north will be cooler and rainier than south....Venice will be for sure cooler than Rome at that time of year. Florence can be warm... April is unsettled weather time, not winter, not spring..you should plan to be prepared for anything!