Honestly, trust your gut on this one. Unless you had significant issues getting pregnant the first time, there is no reason to believe your ovaries are going to shrivel up on your 35th birthday. I (and plenty of other healthy women with fairly normative fertility) have successfully and easily had children after 35. If you were 42, it might be another matter, but you're not. My older son was 4 when we had our youngest. There are advantages and disadvantages to whatever choice you make. If you wait, as we did, you're out of diapers, and then back in them again. But you have a break from "no sleep" for a few years before you dive back into that newborn schedule. If you wait, their daily schedules are a little out of sync sometimes (naps vs no naps.) But you'll also have a few years when your oldest will be at school all day and your youngest gets to be a sort of "only child" for a few hours (whether with you or nanny or daycare or whatever.) We have some close friends whose boys are also four years apart, and they're 12 and 8 now - and great, great friends, so don't worry that they won't know or be connected to each other.
All this to say, don't get caught up in what you think you ought to do, or what you think you should do, or what you thought you wanted when you started all this. Look at where you are now, and where you want to be in a few years, listen to your heart, and you'll know what to do. Good luck.