Just some thoughts from the mom of a 4yo:
Are you charging in any way for the child to stay there? I know breakfast is complimentary, but are you charging an extra person in the room fee? If so, as the person paying the bill, I would have my nose bent out of joint if my kid didn't have a place to sit and eat his meal. A little squished is okay - s/he's four, doesn't need the same amount of space as an adult.
I agree with a previous poster - so far, our current 4yo and his older brother (when he was 4) are/were far too large to sit on someone's lap for a meal.
I also wouldn't object to two seatings - one breakfast seating of 6, then a second one of 3 (mom, dad & kiddo). I'm not trying to suggest extra work for you - I'm just saying it's an option that I, as a parent, wouldn't object to. Or give them the choice: two seatings (with parents & child eating after everyone else) or sit squished up.
The small table is a good idea. My (current) 4yo might or might not like sitting alone at a special table. I think that depends on the child - my 6yo would have *loved* it - he would have felt like a little prince. I might go a bit further though - maybe get some of the cheap but cute melamine plates to make a nice setting. If I were the innkeeper, it would be well worth the money (depending on where you look, $3-5 for a full place setting - plate, cup, flatware) to know that my real plates and glasses would not be in danger of breakage. To be honest, I carry a set of these with us when we travel. It gives me peace of mind. No matter how well behaved a 4yo is, accidents happen..