Being able to see each other is absolutely fine , they just cannot have physical contact outside of their bonded groups.
Unfortunately a 5ft hutch is below the minimum size for four piggies (any area they get locked into (even overnight) needs to meet size requirements) as 5ft is only the minimum for three piggies. Four piggies need a 6ft as a minimum but a 7ft (which doesnt exist commercially) is recommended. Even though it is just for the night, it still needs to meet minimum size requirements. In addition, it wont be long before they cannot go into runs due to the weather changing so they will be spending a lot more time in the hutch making it more important thy have plenty of space.
(the way to work it out is that a 4ft hutch is the welfare minimum for two sows. You then add an additional two square feet for each additional piggy - meaning for three sows a 5ft hutch is needed as a minimum; and for four sows a 6ft hutch is needed as a minimum).
Being double storey hutches unfortunately doesnt count as upper levels cannot be added to the cage size. Therefore all hutches and cages need to meet minimum size requirements on the bottom level only. Upper levels are considered as bonus spaces.
The 4ft hutch is the minimum for the two sows, so is ok for them.
No, the hutches are definitely not big enough for all six together - it would need a 8 foot hutch on a single level to be big enough. They dont exist commercially so would need to be specifically made.