Congratulations.
You have most definately done the hard part. Going from 2 to 3 children, a fourth will just seem to slot in.
Granted childcare will be hard but you have done it before so can surely manage again.
Baby will slot right in, and just go with the flow as no doubt your 3rd child did for the first couple of years anyhow.
All those baby and toddler kisses and cuddles to look forward to again. You have most definately learnt by your mistakes by the time the fourth comes along. You will be a pro and baby will be a much better sleeper, more easy going etc, than any of your other children. At least in so far as the first fourteen months of life are concerned (my dc4 being 14 months).
The washing is already a monumental battle and the fourth doesn't make it any more monumental really.
The fact that your other children are older will make this pregnancy so much easier. No toddlers to lift, get in and out of cars, swings, buggies etc. Older kids can bring drinks to you whilst you sit it out on settee!!
An extra person to look out for you in your old age!!
Whilst I am not too old to cope with my fourth
I do have very willing helpers when we go to the park, when I need nappies or wipes from another room, when dc4 is attempting something dc4 shouldn't be attempting.
dc4 has an old biological mum but 3 very willing enthusiastic adopted mums, whom he tolerates loves very much.
You will cope and dc4 will slot in so well.