This is such a hard situation for you and your hubby, I have been exactly where you are. My DH and I had three, he reneged on the third. I really wanted the fourth but he was like no way. We argued for two years about it then I gave up. I found it unfair and frustrating that it was practically his decision. He kept saying he understood my feelings but I'm not sure men do understand crazy clucky feelings, they don't get that you can't function without thinking about it. Anyways in all of this I was diagnosed with PCOS and was told I could have no more children. They were wrong! We became slack with contraception and fell pg about 2 months after.
My DH has not for one minute blamed me or resented our little girl. He had a Vasectomy done 5 years ago which has been reversed today as we are planning #5....my point being time has helped my DH, having the 3 so close was very full on for him, but the bigger gap between 3-4 was manageable.
Maybe you could discuss when you might want to do it and maybe give him some time? I feel for you, as his wife you don't want to pressure him but you also want him to respect your feelings too. Good luck luv xx