as long as you are comfortable with your fabrication skills, it shouldn't be too difficult. I would suggest getting the seat out and looking it over to decide what you want to do.
I think the easiest thing would be to retain the factory mounting between the floor board and the seat track. If you unbolt the seat track from the seat frame (4 bolts) you can then fab a spacer to go in there. if you get the seat off it will be more clear to you what i am suggesting. If you have power lumbar in the seats you may need to extend the wiring harness dependent on how high you lift them. You are also going to want to be aware of where the stearing wheel will be in relation to the seats. The tilt will only go up so much and you definately don't want to be jammed up when trying to turn the wheel.
4-6" of lift seems like alot to me. I think you will end up in the 1-2" range but i could be wrong. With 4-6" you may also need to think about the seatbelt female end as it would be barried in between the seat and the console if you lift by spacing the seat frame up off the seat track as I suggested. I guess you would also want to make sure you aren't going to be stretching for the pedals if you go for 4-6" of lift. You could always modify the pedal assembly though to reflect the seat changes you make if necessary.
I understand you want to add comfort, just make sure you are not sacrificing safety to do it. Use appropriate materials and fabriaction methods and retain the functionailty of seatbelts and what not.
you may also want to consider replacing your seats with ones from a later model year. You will need to make adapter brackets to get them in though, as '91 is the year that has a different bolt pattern from most of the 1st and 2nd gen seats, 3rd gen is a whole other ball game. That being said the later seats that are 6 way power offer a ride height adjustment that can get you up an inch or 2
best of luck.