It sounds like she thinks you're expecting her to watch the new baby, too.
Are you?
If so, this is an assumption you should not make. Sometimes, grandparents, depending on their age, ability, and health conditions are just not up for newborn detail. It's nothing against the baby. It doesn't mean she won't love the baby or won't want to visit or watch the baby occasionally.
Rather, it may be that two is more than she can handle on a REGULAR basis. Caring for an infant and a preschooler on a regular basis would limit what she can do with your 4 y/o and for her herself. For instance, on days she's watching your son now, she can still run a few errands easily, take him to the park or other child activities, or even get a few of her own household tasks done while at home. With a newborn, even a quick stop for a few groceries on the way home from the park can be difficult. I can't imagine my own mom trying to lug a baby carrier or pull a stroller out of a trunk at this point in her life. While us moms do it all the time, for your MIL, trying to do laundry, make lunches, clean up, feed and diaper a baby, tend to nap times, and entertain a 4 y/o may be more that she can reasonably do at this point.
There will be limits on what she can do with your four year-old as well. If she's used to taking your son to the park, or pool, or story time, things will have to change a bit to accommodate the baby's schedule.
You said you told her that you had "options" for childcare, but did you SPECIFY what those options are? Until you do, she probably thinks you're automatically assuming that she is one of those options, and that probably has her upset.
I can recall after deciding to stay at home with my son, another mom "friend" who decided to divorce her husband to "pursue greener pastures" assumed that I'd take her son home from preschool and care for him while she worked. Um no. No way. I made a conscious decision to stay home to take care of my baby and my family. I was not interested in providing daycare and had to tell her that specifically. I was especially annoyed because she automatically ASSUMED I wasn't doing anything else!
If you have your other childcare options in line, TELL her what they are so there is no more assuming. She'll probably be relieved!
If she's still acting differently after you tell her, I'd just let it go and let her get used to the idea that you're adding to your family. She sounds like a good grandma and a nice person, so give her the benefit of the doubt.
Good luck and congratulations on your pregnancy!
J. F.