Snow Leopard upgrade to Catalina

Catalina requires a minimum of Mavericks (10.9.5), so a direct upgrade is possible through the Mac App Store. Recommend you follow the steps outlined in How to upgrade to macOS Catalina. Whatever you do, do not ignore a Time Machine backup before the upgrade.

Compared to Mavericks, Catalina is huge, slower, and will suck the life out of an older Mac with just 4GB RAM. The operating system will consume almost all of that 4GB RAM, leaving you with slower than appreciated application behavior. And, with a six-year old MacBook Pro, expect half the battery life you get now.

with 4g system memory catalina would be very very slow

I wouldn't upgrade past Mojave (10.14). I'm currently running Catalina and will soon go back to Mojave. Catalina takes away far more than it gives.