Gosh, no, I couldn't do 4 either! And certainly not that close together!
We have ds1 (age 4), ds2 (age 2) and dd (just turned 1). I had 22 months between them both times. My situation is like yours in that I'm at home and dp earns enough for us to live on, pay the mortgage etc.
I cannot tell you when or if to have another, just my own story! I knew I wanted three children and I didn't really want to wait too long as I wanted them all to play together, so they had to be fairly close in age. Also my age was a factor as I was 33 when I had ds1.
I found life changed a lot for me not so much when I had dd (March last year) but in September last year when ds1 started full-time school. I became much more tied then as I had to collect him from school at 3.30 whereas before we used to go out with friends in the afternoons. Also, I missed him loads.
I am the only one of my "crowd" to have 3 so I have felt a bit like the odd one out.
What I am trying to say is that I have found it harder going from 2 to 3 than from 1 to 2. For a start, the kids outnumber the grown-ups now! Also, ds1 and ds2 are both children really now, rather than babies, and I feel like we've moved on from the baby stage. Or would have if we hadn't had dd.
That sounds really awful and it's hard to even say it because it sounds like I don't want/love/appreciate the baby. Which isn't the truth at all. It's just the logistics of looking after 3. When ds1 was 2 and I had a ds2, the toddler and the baby seemed much closer together than the "toddler" and the baby do now. I guess the 2nd one comes on quicker because of the 1st one so the gap seems bigger.
I also find that I'm much more tired than I was with 2 and still don't feel like I am "back on my feet" - whereas after the boys I was coping just fine. But I'm sure that will change as she gets older.
It is harder to find baby-sitters now as we don't have family living nearby and I feel like childcare will be more of an issue if I do decide to go back to work. It's not that we go out a lot, it's just that ds1 has started swimming lessons, etc. now and I have to take two younger kids along with me and entertain them while he is swimming. This is difficult if it's after school as the littler ones get tired and grumpy and it's not really fair on them.
Those are the bad bits. All of them, I think! So just when I've really put you off, this is what I really think:
I would definitely recommend having 3! Yes, really, even after everything I've said. It's hard work but not impossible. She is a lovely baby, a really gorgeous girl. It is lovely for the boys to have a little sister and I know that in a few more months when she is walking and talking things will be a lot easier. I'm sure the gap between her and her brothers will close when she is old enough to play with them and all the hard work will have been worth it. It's worth it already, just sometimes I would like things to be a bit easier to have a bit more of a break (because she is a bit naughty about sleeping in the day)!
Really, this post has gone on for ages, and I've just re-read it and thought it's too negative - but it's not intended that way, I just wanted to let you know what the down-side of 3 is, for me anyway.
Best of luck, whatever you decide to do. I think everyone has a broody stage at some point after they've completed their family, no matter how many they've got!