Right now, by constantly switching the pairs around you aren't creating a bond between the girls.
The first step is to sort out a suitable place for them to live, and currently the Eglu doesn't offer space that meets the minimum welfare standards for 2 guinea pigs, so clearly won't work for 4, and has a very high chance of ending badly with a serious fight. There is simply no space for them to escape from each other and use their 'flight' instinct if there is a disagreement.
The reason we don't seem keen on the Eglu is it simply doesn't offer an appropriate amount of space. I understand it has worked for you for the past 2 years, but it would be irresponsible of us to say it is appropriate as full time accommodation. We try to make owners aware of the minimum welfare standards in the hopes they will take this message on board. However at the end of the day it is up to you.
Ironically another Eglu would cost more than an appropriate sized wooden hutch with covers! If you are prepared to spend the money on an Eglu you would be far better off investing in a decent hutch which will offer your girls the chance to live as a group of 4 with a space for everyone.
Additionally a 2 story hutch will also give you another option if they should ever fall out as you can have 1 pair on each floor.
A well made wooden hutch can also be made fox proof - my rabbits live out year round in a wooden hutch, and we regularly see foxes in our garden.
If you decide to go that route please let us know and you will receive plenty of ideas and support.