Bootstrap relicensed itself with release v3.1 (Jan 2014). Leading up to that, previous contributors were contacted to relicense their changes (see twbs/bootstrap#2054), and new contributors had to agree to dual-license their code under both the MIT and Apache 2 licenses for a period of time. As discussed in twbs/bootstrap#2054 (comment) they did have to revert some Apache-only commits in twbs/bootstrap#11927 and rewrite the needed changes in twbs/bootstrap#11928.
E.g. from their v3.03
With v3.1, we're moving from the Apache 2 to the MIT license for the Bootstrap code (not the docs). We're in the process of collecting permissions from all Bootstrap contributors with code still part of the project to make this happen. For details, please see #2054.
By contributing your code, you agree to dual-license your contribution under the Apache 2 and MIT licenses.
So Bootstrap v4 contains 100% MIT-licensed code (the docs are licensed separately). Some components may have previously been subject to the Apache License 2.0, but their copyright holders have since agreed to relicense them under the MIT license.
If you publish MIT-licensed code you do not issue a patent license in the same sense that the Apache License 2.0 does. In particular, you do not benefit from the protection of the retaliation clause. (Under the Apache License 2, if someone sues you for patent infringement with regards to the licensed work, their license to the patents through that work is terminated.)
However, by releasing your code into the public with an open-source license you are demonstrating an intent for others to use this code. If that is only possible with patents that you own, that might be an implied patent license.