Perhaps my first post was a bit harsh. My first impression was that you had many blessings in terms of your everyday life - your DC, generally supportive DH (even if he grumbles a bit), comfortable environment, and work/study which you enjoy. In a world where so many have so much less, this seemed to be ungrateful.
However, upon re-reading, you also come across as quite lonely.
I think you have to be realistic and realise that in a 20 year relationship with four young DC, the dynamics of your relationship will have changed a lot, and you may not be giving your relationship with your DH the priority it deserves. Yet, at the same time, you may be expecting too much of each other. Looking after the needs of four DC is a huge commitment, so it is not surprising if you and DC have gradually neglected your own and each other's needs over this period of time.
It seems to me as though you both need to socialise more - separately or together - and pursue some interests that are not work related. Is there something that you could do together (subject to childcare) - if there are more external stimuli, there will be more to talk about.
It sounds as though you are both quite bored at present and, with your DH, inertia has set in. He also probably senses that you have developed some contempt for him and are starting to regard him as intellectually inferior (Yes, definitely a touch of the Ritas!). It seems that he has supported you and the family for many years and continues to do so and also helps with domestic chores, so I can understand it if he feels unappreciated at present.
You also appear to resent him because you 'gave up' your career to live with him, as you lived a long way apart. You must see that this isn't fair. You could have carried on with your 'career' if you had chosen, as there were no DCs at that stage. You appear to be jealous of his success despite the fact that you and your DCs derive the benefit from it. Your work may be badly paid and insecure but you are fortunate that you enjoy it.
It is time to make some changes in how you live your lives - not necessarily anything too dramatic, small shifts can make a difference. Could you plan a monthly trip to the theatre, for example - something you could both look forward to - again, it would give you something to talk about.
Good luck!