Gosh what an emotional time for you. Firstly you've done the right thing if he cheated on you, you don't need that in your life.
He is also no doubt full of emotion of some sort and might not have thought clearly about wanting time with his children, but, his immediate actions show you can't depend or rely on him.
It's not his choice for you to have a termination or not, it's yours. I assume you'll be contacting css for child maintenance etc.
Don't make a rush decision, you've got a little time. Make an appointment with your gp so you can start conversations either way, think practically about what needs to happen either way.
With a 4 month old it means you'll have a new born and a one year old if you go ahead.
Pros -
you've likely got all the expensive baby stuff you need
Siblings close together will play together (we'd call them Irish twins as they'd be so close in age)
Cons
You are alone - it will be serious hard work. You'd need available time from someone to help you regularly.
Childcare - do you work? Can you afford two babies in childcare at the same time?
You ex p will probably be a complete twat through all of this.
Cost - everything you then need for two babies, double peaks etc and in life, with only 8 months between them you won't get far on hand me downs.
Practical - do you have enough room where you live, your car - 2 seats and a double peak?
Perhaps continue this list yourself and see where you get to.
Personally - I have a 9 month old. At 4 months old I was ready to do it again, I was so in love and felt amazing. But at 9 months, she's a different kettle of fish and there are hard days now. I personally couldn't do this 6 months pregnant and can't imagine baby 2 when she would be only 1. It would be very very demanding. I don't think I would cope very well.
What is your gut instinct saying at the moment?