HI,
I have a rode video mic, and H4n,
I want to use it such that H4n gets ambience, while the rode gets interview voice.
What else do I need on this setup?

H4n cant use the built in mic, if I attached the rode to the mini jack

Or is this a good idea?

Don't record them together. For the interview you want a track that has a little outside sound as possible intruding onto the speech. For ambience, you only want the background sounds of the location. Then in post you add them together. If you try to record the interview and the ambience at the same time, there will be 'bleed' of the interview conversation into the ambience track, making it difficult to blend them seamlessly later on.

Steve's answer is a good one in terms of optimal workflow for voice/interview/narrative recording, which was what Sigmund was asking about.

More commonly, one might take an "ambience mic plus spot-mics" approach when recording music. I've been doing a series of 4ch. recordings on the H4n these last couple weeks - an M/S stereo mic plugged into the mini jack, and two mics into the XLR inputs. That the recording level of the XLRs is linked is a little bit of a hassle, but, it's working.

You do need to somehow use the XLR inputs as well as the x/y pair or mini-input.

As the OP pointed out, as soon as one plugs into the mini-jack, the onboard mics are disabled. Meaning, no matter how you approach it, the XLR inputs need to have something feeding them in order to record more than 2ch, whether that original 2ch. is the onboard or a mic attached with a mini cable.