I want to put a 4’X8’ platform in my garage to hoist stuff up
If you have a hand framed roof with a non-structural ridge beam and you want to cut the rafter ties, then you will need to add collar ties. The rafter/collar ties are in tension, so you need one or the other to prevent the roof from collapsing in on itself and pushing the exterior walls out. The other option is to add a structural ridge beam for at least that 8' section, but that's not an ideal solution. 

If you have a hand framed roof with a structural ridge beam, then there is a chance you don't need rafter/collar ties at all. It depends on what it looks like up there. 

If you have engineered roof trusses with metal gussets, then it gets a little more tricky. An engineer would definitely need to get involved. 

If you upload some pictures we might be able to help you size it, but more than likely you'll need to contact a contractor or engineer in your area.