Stil having firefly won’t disappear problem with Corona 4 Hotfix 1. Even without caustic solver, without any caustics in material, and noise level at 4.76% it still won’t disappear.
Using less intensive and less sharp light source (e.g. overcast HDRI instead of Corona Sun)
Lowering the overall reflection glossiness of the grass material (something as low as 0,3 - 0,4 should work)
Using Corona Denoiser in “remove fireflies” mode
Using Highlight Clamping in Render Setup > System (not Highlight Compression in the VFB) - but remember this will kill the high dynamic range in your image if you need to save in 32 bits