Your seats won't be sold, you paid for your seats so there will be 3 seats on the plane for you.
Unfortunately as you booked late, the choice is limited. When you go in and view the seating plan, is there just 3 seats left?
Airlines will always try to seat younger children with an adult, this can mean splitting up a family, once the young child is seated with an adult/parent.
You can reserve your seats and then go to the check in desk and ask if they can change your seats, it may happen that there's no shows, or they are aware that there are single travellers willing to swap.
Airlines allocate seats to travel agents, they keep seats that they sell. You will only know what's fully available by speaking with check in staff.
Not everyone pays extra for their seats, seats like priority boarding, extra leg room, seats up front, bassinet seats are extra, people book and pay for these seats. The other seats are free for all, people will have reserved these seats as they may have a preference as of where to sit, ie isle, window, near toilet etc. You may find someone will have reserved a seat with no extra fees but would be willing to swap.
If you don't have any luck at the check in desk, you speak to cabin crew, explain your situation and they will ask passengers if they're willing to swap. You might not get seated all together but they will ensure that your 4 year old is seated beside your other adult.
The only airlines I ever had issues with were Ryanair and delta, both expected my disabled dd to sit in a separate seat, I normally book and pay for priority boarding but on these 2 occasions one flight was last minute and the other was just a fuck up from start to finish. It got sorted in the end, but it involved a lot of begging and dh paying a man 50 quid to swap his seat. Most passengers are willing if travelling alone.
You might need to expect not to be seated all together, the crew will do their utmost to seat your 4 year old beside your other adult.