I agree with all the above, if it were me:
1. I'd put all my 4 weekly prices up; not a lot, just a bit, an increase for every customer. This gives you more income, but more importantly, also offers an opportunity to 'offer 8 weekly cleans', for a further slight increase. Don't go 50% more (not on existing customers) I think you're pushing it a bit. Negotiate a price that is 'fair'.
2. Look at the 'status quo'.
You may have lost some customers (due to the 4 weekly slight increase).
Most customers would have accepted the 4 weekly slight increase, and some customers will now be on 8 weekly at an even more favourable rate. If you're good then you could have converted say 40% to 8 weekly.
3. You need 2 vans. 1 man in each.
My van was in for a service yesterday, so I helped out a mate and we did 2 up on his round. You can't earn twice the money as 1 man. We did have a good laugh though!
4. Any new customers, just offer 8 weekly at a higher price, but again, a price that is fair and reasonable. You'll have a shortfall of work now, so you still need to build the round.
You will find that mixing 4 and 8 weekly will be a pain at times.
I only offer 8 weekly but have ended up doing 4, 6 and 12 also. I'm a soft touch and they are lovely people and it's only for about 20 custies. As much as they are all lovely custies, it does annoy me, trying to fit them into my 'perfectly planned schedule'! lol.