Thanks for the prompt response.
I agree, the cage looks to be small for 4 budgies.
1 budgie is temporarily staying with our budgies as he belongs to my sister as she is out of town. So it would be 3 budgies when it leaves.
Regarding perches, they are plastic perches which came with the cage. So I guess, I will look for different natural perches for them.
My budgies like to play with the nests though. They sit on top of them and try to peck it.
I will remove them if you say so. There was a wooden swing inside the cage, but it was getting very crowded with a lot of toys, thats why I removed them.
I will also go through the links you shared. They look to be very helpful.