Clarification needed on playing 4 equidistant strings

I've been asked to help a beginner get started on dulcimer and discovered she has 4 eq strings.  I read the threads related to this and Heidi Muller's suggestion to tune DAdd and let the inner d string just drone out.  Does anyone have any other suggestions or tips?  I play 3 strings in DAd and am not sure how that extra  eq-distant string from the melody string will affect chord playing.  Dulcinina