Neither the 1D X nor the 5D Mark IV are "weatherproof." Canon refers to both as 'weather resistant'. If you use either in the rain and water damages the camera it is not covered under warranty.

The main advantage with regard to weather resistance any of the 1-series cameras have over the other recent higher end models such as the 5D Mark IV and the 7D Mark II¹ is the integrated vertical grip. If you're going to put a battery grip on the 5D Mark IV or any other Canon body that is not a 1-series, it increases the number of possible failure points for allowing moisture to get past seals and o-rings and into the camera.

But then again, if you add a grip and a second EP-L6N to a 5D Mark IV, the combined weight at 48 ounces is only 6 ounces less than a 1D X at 54 ounces.

Roger Cicala, founder of lensrentals.com and well known blogger, says the 5D Mark IV's weather sealing "appears pretty robust." In the same blog he compares the level of weather sealing for the 5D Mark IV to the 7D Mark II.¹ He also notes that it is improved over the 5D Mark III.

Of course, Roger also says:

OK, as you all know I don’t take cameras out in the rain without plastic covers. Because we write off ‘weather resistant’ cameras for water damage every week. But I know you young people live life in the fast lane and like to take risks.

¹ When he did his first tear down of a 7D Mark II, Roger Cicala said it may be the best weather sealed camera he had ever seen. Presumably that would include the 1D X that had been out over two and a half years when the 7D Mark II was released.