Are you married? You call him your partner. I'd be thinking of a future if you were to split. Who owns your home? Would you manage alone physically, mentally and financially?
Unless that dream job is very secure (none are, especially without under two years under your belt, regardless of pregnancy being a protected characteristic) and very well paid, would you need any state assistance as top up? Only two of your DC would be provided for, making life already hard enough with three DC.
This is your choice alone. Your partner knows you've had one pregnancy he didn't want and I assume he knows of your desire for four DC. Why would he not make sure this didn't happen with you by having a vasectomy, unless you didn't want it or he's hedging his bets? Either way he must accept that pregnancy is a chance he took.
You say you want four DC but the reality, (even if the baby is born healthy), is that you are prospectively making yours and your DC's lives more difficult and they should be your first priority.
My friend was in your position. Her partner owned the home and chose to make her leave. Number 4 ended up being numbers 4 and 5 with one having extra needs. I do not think she would ever regret the DC. She had to give up work. Even without her partner leaving her life would've been far more difficult than with three DC, but she's now a single mother of five surviving on benefits in a too small rented home.
What do you think an extra DC would bring to your life that three DC doesn't provide already? At this present time I couldn't go through with it.