@Matthew1870 a few thoughts
If you have a flexible or moderate policy then guests can book your place and then just before/a week before they can see if anything better/cheaper is available and leave you in the lurch. This is more likely to happen in off season or where the market is saturated. We have a strict policy and are happy not to get the odd booking because we know the ones we do get are firm.
Re the 8 person property. If you charge extra guest fees then you will be forever worrying about how many guests are there. Also even if only 4 book they can use all the bedrooms so you save nothing on cleaning/heating/cooling/laundry. A far better idea (I would say) is to have a 4 bed and a 2 bed listing (same as 4 bed but with rooms locked off) for 8 and 4 people respectively. Link the calendars and block the 2 bed listing in high season to force people to book the 4 bed.