Just want to confirm from what homedepot told me .... I asked if the
dimensions of 1"x4" pine was the same as 1"x4" hardiplank. He told
me only the thickness is different. He said pine is 3/4" thick and
hardiplank is 1" thick. Is this correct?

I plan on replacing some outside house trim originally pine now with
hardiplank. I want to replace a vertical 1x4 trim now in pine with
hardiplank that will meet just under the soffit to a horizontal 1x4
pine (butt together). I'm thinking no one will notice a 1/4"
difference in thickness 2 stories high just under the soffit and
painted.