This totally depends on your hair texture. for Caucasian types it would be out of the question but some 4s would drink it up like its a light snack and then ask if you want to put on a little extra to enhance the slight gloss just a bit. I know my partner who is a 4c uses castor oil on his hair and It is about the ONLY thing that will keep his hair from looking totally parched and completely dried out and then it still dries out in a few hours. However when I use it on my fine 3b/c my hair looks totally wet and way too thin. then again, my hair is already on the thin side so it doesn't take much to make it look like I have 2 strands on my head.
So hairs respond totally differently to different methods of moisturizing and protection products. Also amount has a LOT to do with it. I can use the castor oil as a leave in product too if I put just a few drops literally in my palm and evenly distribute it in my palms before I put it on the ends first and then evenly through out.
So I say "whatever works"
if it works for you then you shouldn't care about Sally whose hair is too fine for it. She uses the viscosity and oil/moisture balance that's appropriate for her and you likewise.
REALLY, I think that you should try to switch to heatless methods as much as possible. My grandmother had 4a/b hair and she got relaxers and wet sets exclusively. Yes she did use coconut oil, jojoba oil and some others to keep the hair soft and shiny but she never used heat at all. Her hair grew to about below shoulder blade length and stayed there all of her life looking shiny and healthy. But she ONLY used relaxers and wet sets on huge rollers. The Dominicans kind of go this route too. So I think that would really be your best bet instead of looking for a heat protectant. I would try to work into heatless styling. MAYBE only use the heat around your edges when its time for a touch up after the wet set is dry.
SO.... instead of flat ironing why not look up "silk wrap" on youtube. It is a technique that can be used on wet hair to straighten it using the head as a big hair roller. You stretch the hair straight around your head and tie it up with a silk scarf overnight.
Also look up hair wrap techniques on youtube. Because you can also use a variety of other wrap techniques (depending on hair length) on the hair every night when you go to bed to preserve the hair in a straight state so that no heat is needed for styling in the morning. Rather you wake up, take off your silk scarf and your hair falls down around your shoulders silky, shiny straight and healthy like it was just flat ironed .... only it actually feels healthy instead of fried. (for long hair look up criss-cross wrap)
Ive just started this regimine myself and let me tell you... I am floored and totally in love with wrapping my hair instead of blowing it straight. I never used flat irons. I use a japanese straightener every 3 months and wrap every night. 2 nights a week I wrap with plastic cap and tight scarf on top for a green house effect. Im trying to grow from arm pit length in the back to as long as I can go. I wont get there using heat though.