In Generation 4, Cherrim used to transform to it's Sunshine during Sunlight (weather) in battle, without the influence of Flower Gift. This means, in that gen, Cherrim was able to transform even without that ability. It was from Gen 5 Cherrim began to transform using Flower Gift.
(Proof : Using a manipulating software, change the ability of Cherrim in any Gen 4 game. Enter it in a battle and use Sunny Day to activate sunlight. See what happens. Do this in any other Generations. You will find that it won't happen!)
So, Sunlight alone was the cause of transformation of Cherrim in Gen 4
Source #1 : My experience