I dont know your back story and forgive me if I am jumping to conclusions but.....
They say abuse often doesnt show up until there are children, especially if the mother gives up financial security and is then dependant. The balance of power shifts and the abuse can start to sneak up.
Its bloody hard changing from someone who works, is viewed as an equal, and has 'value' in the world (and I use commas because 'value' does seem to be attributed by society as anyone who brings in a wage/pays tax) into someone who may feel at a loss and isolated bringing up a baby without any support, and who's status has dropped through the floor. To then add in another baby before the dust has settled, meaning no end in sight, and possibly the loss of a job that had maternity pay attached(?) - its easy to drop into depression.
A marriage with the power shifting, where suddenly you have fallen back into the 1950s and taken on all the drudgery of housework/night time feeds, no time off, whilst the partner seems to have his life actually IMPROVED (absolved of all household duties and suddenly elevated to Lord of the House) creates alot of tension, even IF the husband is kind and helpful. If he actually starts to throw his weight around, take advantage of being able to swan off willy nilly, control the finances, and its a recipe for unhappiness. What may look like PND may actually be a reflection on the marriage.
So now, after 4 long years of suffering this, exhausted, and berating yourself for being short tempered/uninterested/irritable (and its easy to be the calm and lovely parent if you can drop in and out at your own convenience as he can, if the rest of your life is bolstered by adult conversation, career, etc) you have gone back to work and you love it. You love the financial freedom of it. You enjoy the challenge. You enjoy feeling like a person again. And why wouldnt you.
Its obvious that you love your children - the little you say about them reflects that. I dont think that you want to leave them, but I think you want to be working too. Its fine. Many people feel the same - doesnt make you a bad mother.
But Im guessing hubby isnt too impressed. From you carrying everything for the last 4 years you are expecting him to pick up the slack like you have been doing. To arrange pick ups between you. To have some spare time yourself. To take a share in running the home and taking responsibility for everything. It has inconvenienced him. Im guessing that he wants you to give up your job and return into the home, that he belittles your job because the money it brings in is not comparable to his wage, and he resents having to pick his dirty pants off the floor. Irrespective of what the job does for you as a person, or what prospects it may have.
So he is trying tactics to pull you back into line.
I may be right off the mark, and apologise if I am.