I'm also a righty with a 4 year old lefty. My approach is probably less than conventional, but it is what is working for us.
I don't "teach" writing. When she wants to color or write I will watch her and help her place her fingers to hold her pencil or whatever else she's using the correct way, but other than that I leave her to figure it out on her own. She needs to figure out what is going to work for her after-all.
She doesn't like having lines and being told to draw a specific letter on them. Instead, when she wants to try to write something specific she prefers for me to write it first with a highlighter for her to trace.
When we're introducing new letters I will sit on her left side and use my right hand to guide her left hand, but after the initial introduction I usually just leave her on her own to figure it out.
She's not really interested in writing right now, but I am gradually seeing more and more interest as her fine motor skills become more coordinated.