[EricH]

You know, there's a couple of "advanced" topics you may want to research on your own time.

Static loading and dynamic loading. The 90 lb is likely a static load situation. Your 150 lb robot is most definitely a dynamic load situation.

Static: Not moving, no "extra" forces. (Just to pick an example, you park yourself on the end of a rope rated for your weight and very gently remove the support you're standing on.)
Dynamic: Motion involved, "extra" forces from various factors. (Going with the previous example, you jump up and grab onto the same rope--or use it to stop a fall, your choice. BTW, don't try this at home, I don't expect it'll end well.)

Long story short, something that's rated at 150 lb might not hold up when 150 lb is lifting itself--you might want 300 lb, 450 lb, even 600 lb. Maybe more. And 4 strands of 90-lb rope may in theory be 360 lb, but even the rig technique you use can drop that.

I could go more in-depth, but this has the makings of a) off-topic and b) fun offseason learning project. Talk to your (mechanical) engineering mentors about trying this sort of thing out or running the numbers.