@Lee: you have the same Comcast plan i previously had. your line is provisioned for 120 Mbps down, and 6 Mbps up (20% overprivsioned for 100/5 plan).
i live in an area where, because of the pandemic and everyone working from and doing school from home, the nodes are saturated so that at midday and body using Zoom/webex/google classroom at our house, i was speedtesting as low as 1.8 Mbps.
if your local node is not saturated/overcommitted and you can get the promised 5-6 Mbps upload, you should be fine for 4 simultaneous sessions. in Zoom you can keep an eye on how well it's working by looking at the Statistics under Preferences. it will show any packet losses and actual bandwidth used.
but you have a good plan in place...if you get reports of "connection unstable" from Zoom, then the simple fix is to go for a higher tiered plan. i don't know if you are under contract right now or not, but depending on where you live, you can probably negotiate a decent price for a higher tiered plan.
YMMV may vary (and i don't know your budget), but in SF Bay Area, when i called to cancel my service (i did), they offered me their 600 Down / 15 Up plan for $60/month for a 1 year commitment. if AT&T Fiber is available, you should look into their promotional deals.
we thankfully had an alternative (a small fiber to microwave LOS tech) which is now giving us 700 down / 220 up with no contract for less than that Comcast offer i mentioned. (no AT&T fiber available at our house)
anyway, i digress. the biggest issue with Zoom will be your total upload bandwidth. if you can consistently get 5-6 Mbps, in my experience, you should be fine with your existing service.