I believe they only push notifications every hour or so, or you get them sooner if you’ve favorited the listing.
4pm isn’t the end of the day at all, if you want to have an approx cut-off time, I’d go with something more like 8pm. THS isn’t a 9-5 thing, with it being online, plus some pet parents work or have active lives during the daytime, it just depends.
Also if you sending messages via THS the system has a time delay (it varies), so they’re unlikely to get your message instantly.
The limit of 5 is THS, but if a pet parent doesn’t think someone is suitable (or thinks they can find better), then by turning someone down, it will free up a space.
THS places the limit so it’s fastest 5 to apply. Can you open up your search area & also get a few reviews from local sits near to you before heading for Italy? #bellissimo
Either party can initiate a video chat. 
I browse the forum everyday for new posts.