Don't worry. You're still four years out; you'll re-run your calculations a million times before the big day arrives. Things will change, possibly a lot, during that time, but you'll be following closely and will adjust as needed.
Are you in the U.S.? If so, the numbers you gave don't appear to include social security, which means you are being extremely conservative, especially considering that you're planning on a $20k spending buffer at a 4% WR. If you're working until you're 52, you should have a substantial SS benefit coming when you hit 62 - 67. If you and your spouse are both currently employed full time, it is likely that your SS payments will more than cover your $35k basic spend. I would encourage you to re-run your numbers including expected SS benefits. You can reduce them by 25% if you're nervous about the future of that program. The results might have you moving your FIRE date closer.
Disclosure: I haven't FIREd yet, but if things work out in the health care arena I should be going within 6 months.