Boost converter with input-referenced output
If I understand your question correctly, I think this is the basic topology you're wanting to impliment:



You can replace the control input &amp; Q1 with your monolithic IC, of course, and you can (depending on your selected IC's structure) measure your voltage across the output pins here (capacitor not shown, but obviously necessary) to achieve your stated goal of 4VDC output with isolated ground. Just remember to never, NEVER connect anything to this circuit that can accidentally communicate with 'true' ground, as that 3-5V ground offset could very quickly cause damage.