Hi aminah,
This makes sense for a mattress where both sides of the mattress were layered the same. the 3 layer combination would feel firmer to you because of your lighter weight but if you add an extra layer of medium then the extra thickness in the top 2 medium layers would make it softer for you and still be fine for your husband. The reason for not using a soft over a firm is that heavier weights could “go through” the softer layer and feel the transition of the firm layer below it and this may be uncomfortable.
There are also manufacturers in the online members list here which can build a 'split layer" mattress which means that each side can have its own layering so you could use a 9" mattress with your side being S/M/F and his side being M/F/XF.
Some of the better options and possibilities in the Detroit area are listed in post #2 here. the better options in the Grand Rapids, MI area are in post #273 here (including one of the manufacturing members of this site).
You have some very good options in your area.
Phoenix