Conditional Formatting - How to use formula type???

Every month I have to manually modify this table (which is time-consuming).

If "RVUs YTD" is greater than the "RVU target", I want a green checkmark and if its less then I want a red X. I have already achieved this as you can see in my screen shot, but, I have to modify these each month manually (these values increase each month) .

In my second screen shot, if I select "type" and make it "formula", how can I achieve to do this automatically?

What do I type in the value box so that if B4>D4 it will make it green automatically?

Thank you

*I have attached the spread sheet.

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Thank you both, allow me to clarify ...to make things eaiser, let just assume I input these number manually.

RVU Target - This is the number of RVUs a person should have.
RVUs YTD - Is the actual number they reached, it can be more or less than the target.

My goal is, I want a conditional formatting setup so that any time column B is higher than column D it shows a green check mark and if column B is lower than column D its a red X.

I figured formula from the drop down may help but I have no clue what to enter there. From Fords comment, looks like it may not be possiable

*Thanks Ali for the link but I am still confused