Wedding party!

You can have uneven numbers nothing wrong with that sooo many people do it!! just add her and dont worry!

We had a similar issue.
Rather than add people, I've asked my friend to come get her hair done day of with my sisters (my bridesmaids), Mom and I. She understands the situation and was all for it (side note, she's been a bridesmaid a few times and is excited she can reuse a dress lol).
It was easier than trying to add people just to even it out- I've been to a couple weddings like that and there was drama amoung the whole wedding party because of it.

An alternative to the uneven number party is your FH having a groomswoman. If he has a really good female friend, no reason she can't be a part of his party while you get a fifth.

When my uncle got married, he had all of his brothers (my dad plus 2 uncles) as well as his best man, and my aunt had her two bridesmaids plus his daughter (my cousin). She had no official maid of honour, but what they did was the best man and my eldest uncle walked down as a pair with her bridesmaids, but my cousin walked down holding both my dad and my other uncles arms. Made for some sweet photos with them walking as a trio

You can definitely have an uneven wedding party, so I wouldn't worry about adding someone who might feel like a placeholder.

Uneven numbers is totally fine. A good photographer can still take good shots, basically that is what the fuss was about long ago with the even numbers. I will have 2 or 4 depending on my out of town sisters traveling her. My Fh has 5 and wants to add another. I am perfectly ok with uneven numbers and I am sure our photos will come out beautiful.

Like Rayanne said, there's nothing wrong with uneven numbers. I'm also not a matchy matchy type person though so it wouldn't bother me in the least. I'm also not a huge traditionalist in many ways, so if you and FH want his dad to be apart of the wedding then go ahead and ask him! Even if he declines, I'm sure you thinking of him will mean so much to him. The father of the groom doesn't usually have a major role to play in weddings, so I think it's a great idea that's worth considering

You can have an uneven number of people in your bridal parties. I dont think parents are usually in the bridal party. You could have a groomsman walk each bridesmaid down and the MOH walk herself down.