"I'm thinking ahead to when youngest will understand..."
Why do you think it will be the same invitation when the youngest is old enough to understand?
They might take both of them then. Or take each individually.
You're just inventing a problem here where there is none.
The small toddler you have won't understand, so imaging how hurt he would be if he did is ridiculous.
"but hell yes I'm selfish and spoilt if taking any breaks that are going is an indicator of those traits"
No, you're selfish and spoilt because you're griping that the invitation for your 4 year old doesn't offer you enough of a break.
There is a break going - overnight with just the toddler to mind, but that's not enough for you.
I do sympathise with having GPs nearby who don't seem to want to see the children often :(
Which is why I think getting in a snit when they do reach out is so absurd. Let your 4 year old go and have some top GP gushing and spoiling time. Toddlers are more trouble than 4 year olds, but they go to bed earlier (hopefully :) ).