I would dump the Sigma lens and the 75-300mm (I'm assuming the latter is the "III" without USM or IS, which is Canon's cheapest and least capable telephoto zoom).
You won't get much $ for them... But, hey, every little bit helps. Keep the 18-55mm for now, as your wider lens and a relatively small and light general purpose "walk around" lens. If you use the 50mm for low light work and shallow depth of field effects in portraits, by all means keep it too. Assuming it's the 50mm f/1.8 "II", it's also not a pricey lens and is compact and lightweight. (Eventually you might consider upgrading the 18-55mm too: If you want a faster lens, get the EF-S 17-55mm f/2.8 IS USM... Or if you want a bit wider lens, get the EF-S 15-85mm IS USM... either would be a very nice "step up". Or if you want really wide, the EF-S 10-18mm IS STM is inexpensive, relatively small & light. Or, you mention shooting macro... get the Tamron SP 60mm Di II lens... it's one of the few macro with a larger f/2 aperture, so could replace your 50mm f/1.8 as a portrait lens, too.)
Between the 70-200mm f/2.8 IS USM II with 2X III Extender OR the 100-400mm IS USM II... definitely the latter.
Currently the 70-200/2.8 II and 100-400 II are selling for the same price: $1800. But by the time you add a 2X III Extender to the former, you have spent an extra $429. And for that you will get less image quality and heavier rig that doesn't focus as closely. The 100-400 II is a better option that has superb image quality throughout it's range and focuses close enough to give you nearly 1/3 life size, and weighs about 3.5 lb. The 70-200mm alone weighs a little less... four whole ounces... about 3.25 lb. But once you add the 2X III you are close to 4 lb. total. I wouldn't consider using any other 2X with that lens, for best possible image quality.... though it won't be as good as the 100-400mm without any teleconverter.
By the time you add a 2X to the 70-200mm you have an effective 140-400mm f/5.6 throughout. The 100-400mm is f/4.5-5.6 variable aperture... not exactly a "fast" lens, but it maintains f/4.5 to 134mm, f/5 from 135mm to 311mm, and only drops to f/5.6 from 312mm to 400mm. So, at the focal lengths up to 311mm it's 1/3 to 2/3 stops faster than the 70-200 + 2X combo.
Incidentally, if you ever trade up cameras, many of the newer models (7DII, 80D, 77D, T7i) can autofocus the 100-400mm WITH a 1.4X teleconverter. So you could potentially have up to 560mm effective focal length, longer than would ever be possible with the 70-200mm + 2X . However, the 100-400mm II + 1.4X also ends up a bit over 4 lb. total.
I shoot with 100-400mm hand held a lot. It's only 1/4 lb. heavier than the 70-200/2.8 II (I use the earlier version of that lens as well as the smaller/lighter 70-200mm f/4 IS USM... but both those 70-200s are getting a lot less use since I got the 100-400mm). 100-400mm does start to get heavy after a while. So I often use it on a tripod with gimbal mount. Or at least on a monopod. Most people who use lenses like these for anything more than half an hour to an hour shooting will likely want to use a tripod.
One thing about the 100-400mm II is that it's tripod mounting foot is a neat design, but doesn't work very well with Arca-Swiss style quick release lens plates. There are replacement tripod feet available though (Really Right Stuff, Kirk Photo, Hejnar Photo and others). Those have built in Arca-Swiss style dovetail and work well with compatible tripod heads.... or are essential if using a gimbal type head or adapter. Those replacement tripod feet add around $100 to the cost of the lens (but take the place of an Arca-Swiss plate that would cost $35 to $50 anyway). I hope Canon takes note of what Tamron has been doing lately.... designing their lenses with tripod mounting rings to have a built-in Arca-style dovetail.... no lens plate needed.
You can see for yourself that the image quality of the 100-400mm is better than the combo of 70-200 & 2X:
https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=972&Camera=979&Sample=0&FLI=4&API=0&LensComp=1198&CameraComp=979&SampleComp=0&FLIComp=5&APIComp=2Even though the 70-200 II and 2X III work better together than any similar combo that Canon has made to date, the 100-400mm offers better IQ, is faster (larger aperture), is closer focusing and weighs less.
Read the entire review of the 100-400mm II here:
https://www.the-digital-picture.com/Reviews/Canon-EF-100-400mm-f-4.5-5.6-L-IS-II-USM-Lens.aspxThe 70-200mm f/2.8 II review is here:
https://www.the-digital-picture.com/Reviews/Canon-EF-70-200mm-f-2.8-L-IS-II-USM-Lens-Review.aspxWhat will really stun you is comparing the IQ of the 100-400mm II with that of your current EF 75-300mm III:
https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=972&Camera=979&Sample=0&FLI=0&API=1&LensComp=776&CameraComp=979&SampleComp=0&FLIComp=1&APIComp=0Both the 70-200 and 100-400 are very well made... pro quality.
I mostly use my 100-400 II for sports, but occasionally for a bird, too. This kildeer didn't like me being close to her nest. These were hand held at 400mm and wide open at f/5.6 (on 7DII, an APS-C camera like your T4i):
With a large lens like either of these, you might want to add battery grip BG-E8 to your T4i to improve balance with the heftier lens on a relatively small, light camera. The grip also gives you a secondary set of controls, which are nice to have when shooting in vertical/portrait orientation. BG-E8 are still available new and widely avail. used for about half price (it fits T2i, T3i, T4i and T5i too.... T6i and T6s use a different grip.... and, oddly, Canon has not produced a similar grip for T7i or 77D, neither of which are designed to accommodate a grip either.... the 80D is the "lowest" current model capable of being fitted with a grip).
Thanks for letting us help you spend your money!
I would dump the Sigma lens and the 75-300mm (I'm ... (