Sadly your vehicle has only two tether anchors and with forward facing seats it is highly recommended* to use the top tether. If it is at all possible it would be recommended to put one of the twins in the third row so that you can use tether anchors for both of the older twins' seats (one in the second row and one in the center of the third row).
The second bit of bad news is that the third row is extremely tight and difficult to fit 3 children across. If your only concern was fitting the seats, then you might have good luck with something extremely compact for your 8 year old (e.g. the Bubblebum) on one side of the third row, then a narrow harness for one 4 year old (e.g. Cosco Finale**) in the center of the third row, and then the infant seat on the other side of the third row. The trouble with this setup is that the way the outboard seat belts are set up makes them generally not play nicely with boosters. I have heard that the highback Evenflo Amp is a good choice for outboard, and though it's not a wide booster per se, it might be hard to fit with your 3 across. (The good news is that if you are going to put the other 4 year old in the second row then pretty much any seat should fit there. Depending on what you have for him/her now you might be able to continue to use that seat.)
Another option could be to put one of the 4 year olds rear facing in the third row if you are uncomfortable having one of the infants back there. It will still be tricky to find a combination that will fit 3 across, however.
*If you are in Canada the top tether strap is REQUIRED for all forward facing harnessed seats.
**Note that if you decide to NOT use the tether strap then you can't get a Finale as that seat requires that the tether always be used.