I have 4 kids ages 11,10, 8 and 18 months. We were done with three and perfectly happy about it. Apparently, there was some pranks being played up in heaven, because we now have 4!! LOL! I will start by saying that I am still coming to terms with it. You would have the advantage of no big separation between #3 and #4. I was happy to be done with diapers, naps and such. I think that was the hardest part for me.......I had them all in school and was excited about the next phase of my life. Anyway, I will admit that it is extremely challenging. My oldest is a super athlete and plays sports (including the travel teams that involve a lot of going, going, going). Then, I also have the other two in guitar, volleyball, cheer, football, etc. The older they get, the busier it is. It's hard enough when they're little and all the chaos that goes with diaper changes, illness, naps, tantrums, etc. When they are older it's just as busy, but in a different way. My husband works until about 6:30-7:00, so most of the running is up to me. I often feel bad for my toddler because she has to be on the go with me and there are days that she is in the vehicle more than she is out of it. Homework help can get challenging, for sure. Last night my husband was out of town, so I was on my own. My 10 year old and 8 year old both had homework questions, I was trying to fix dinner, the dogs were wanting to go outside and the toddler was overdue for a bath. Yeah, I was about crazy. That is fairly rare, because my husband is home in the evenings. It's always a juggle and time management. The other hurdle we are facing is the finances. Kids are dang expensive!! I have always been a SAHM and have felt very lucky. However, we are feeling the pull more and more every day. We live frugally, but adding just one more kid has been noticable. Then there's the fact that we'll have 3 in college at the same time. YIKES!!!! Of course, they may take different courses, like military or technical school, but it's a possibility. We have no family in the area, and although we have good friends, it's a huge challenge to be able to do things with the kids, like go to a movie. Your kids are getting to, or may already be, at a stage where you can do these things. You can go to a movie, to an amusement park , etc. and the kids can enjoy most of the same things. Adding a baby will delay that, as well as future outings. There are many more places we want to go, vacations we want to take, etc., but having a little one really has put that to a halt. I see it already, my oldest is wanting to do more with his friends, so it won't be long before our family time will dwindle. I often feel like we are missing the boat on certain things because of the toddler.
Ok, now for the good stuff........My older kids absoutely adore their little sister. SOOOO in love. This summer, they had a blast with her at the pool and playing in the yard. I ask them sometimes if they are upset that we can't just take off for a movie and they say they'd like to be able to, sure, but having her is better. It's probably mostly MY guilt! :) I am the oldest out of four, and some of my favorite memories are of when my sibs and I get together. There are so many fun times that I often think the more the merrier!! Big families are special and my kids will hopefully remain as close or closer than they are now. We've got each others backs, ya know? Raising four kids often makes me feel like I can handle anything. The daily challenges have made me a stronger, more organized person. I do worry about making sure each kid gets the attention they need, but I think it's mostly just the mom's guilt-thing. The kids never complain about it and are so very well adjusted and smart. Just by how our activities go, there are days here and there, where it's just me and one of the kids. You have to take advantage of those moments and do something to make it special. I'll run through DQ for a cone or buy them a little something to make the day(or hours) we have just the two of us, a special time. My husband does the same. He may have to take one with him on errands and does the same thing. It's not an every week thing, but I'd say every month to two months, we take advantage of it. Like I said, I grew up in a family of 6 and I don't remember ever feeling neglected or ignored. My mom had it worse than me, with a pretty absent husband.
As you know, it's up to you. I think if you try to imagine your day to day with a baby, you'll be able to decide if it's right for you. Think of grocery shopping, taking the kids to school, coordinating naps, weekend activities., etc. You'll never really know until it happens, but you'll get a pretty good idea of how difficult or do-able it may be.
Good luck to you!!