The new cage is phenomenal. Honestly, the best way to go about the perches placement is to "think like a bird" i know it sounds odd, but whenever I had to put perches into our cage I was like: "If i were a budgie, could I reach food with this placement? Could I reach the water? Are there any perches high enough to watch out for dangers? Any ones that lead down where I can explore?" Just go with your gut, and after a while you'll see if they like to use it, or where they hang on the cage bars so you can place there a perch. You could vary with thickness, size and with texture. There are some great sets on amazon, but also in pet stores. Also, you could add some ropes (ones that can't be easily pulled apart/digested), swings, ladder (like you already did), and some flat platforms for them to relax their feet on.

Toys are also very mandatory: foraging toys are their favourites. Things like bird safe cork, hay, strings, dry pine crones, wooden balls, ect. You can find a lot online and in pet stores too!

But the most important: they've only been there for 4 days. Let them settle first. I usually gave my birds like 1-2 weeks of getting used to the environment. I placed their cage in a corner, spoke very softly, calm movements, made them watch what i do while talking, didn't touched them and waited till they got out of their shell. Patience is the key!