Hi Anne, I used to have a D300 and now have a D4. The D4 is a machine. You'll see a significant difference in auto focus capability/subject tracking, color rendition and overall image quality (in a marginally larger image size).
Of course, the biggest improvement will be in low light/high ISO capability (and some dynamic range). This camera really sings at ISO 1600-3200, so when coupled with a fast lens, you can literally put away the flash.
On the downside, it is huge and heavy when compared to the D300. I suggest renting one from lensrentals.com before putting down the cash.
Is it a better camera – absolutely! Will you get more keepers in moving and/or low light action—without a doubt! Will your photography improve with the D4—I suppose the answer to this depends on whether or not the D300 was limiting your creative options in some way?
The D4 is just a tool and nothing more. If you found yourself limited in low light situations or wishing you had more firepower when tracking moving subjects, the D4 could certainly help.
Will it make you more creative – well the answer to that is generally no, although low-light shots without unnatural flash lighting to ruin them can certainly open up new doors of creativity—from night photography to photography taken in a dimly lit gymnasiums.
In some cases, the D4 can actually set you back a few clicks before you really get the hang of it, because it’s got a lot more bells and whistles than the D300.
In no way am I trying to talk you out of the D4. I think it’s the best “all-around” camera Nikon has ever built. But it’s not without its limitations and caveats. As long as you know what you’re getting yourself into and why, then the D4 is a no-brainer.