depends what You consider being 'enough'.
Follow the project from early days, been on of those who entered from beginning. Added my machines when they are idling and keep them there while I'm not using. doing this more or less since the beginning and have no regret spending some extra try it. Love the idea.
if You plan to live worry free life from submitting single 4090 machine, then it's not going to happen. However, if You have no rush, wait to the moment RNDR value rises, sell some that You mined, I can say for sure you can upgrade Your machine to better one down the line without having extra expenses.
keep in mind that as You go, You will need to keep it online, keep it ready, so You will have constant power draw - these things sum up. Not even talking about hardware failures, driver issues,..
as for load in general, it depends a lot on community and the demand, but in any case we are not looking into 100% or even 10% load. However, as with all the things it also depends on specifics: You can combine running RNDR jobs and when it's idling, do some other bits (mining), some rent those machines for AI development and such if You wish to make more money from Your investment.