This is a good question. I'm not quite sure if or when v99.4 will come out, however I do believe some very big things are in the works. Two or three elements are in development (but are not final, and are subject to change or abandonment) by a TPT dev and a user. Reactions between existing elements are also in the works and being considered. Many of these things have been in the makings since before 99.3, and may not be finished before 99.4 or 99.5 or anything. I believe the hope is that these will be in the V100 update or earlier. 

So yea, some big stuff is in the works, and there isn't a lot of small stuff that I am aware of. There are very few known bugs to fix. I am a bit confused as to why you would want v99.4 to come out soon if it was just a minor update, but rest assured, things *are* in the works. 

Love Ex Nihilo btw. That and one of jona-hungary's saves were what inspired me to start making infinifactories.

The elements under development are BASE and PAPR, and maybe EPPR (electronic paper). EPPR and PAPR may be merged again tho as they were originally one element. The reaction that got added was one I made, 5gas+xgas+rime/fog=>acid (life=50+5x) upon condensation. This was based on atmospheric pollution creating acid rain, and enables the renewable creation of acid.

Ah okay. I saw this "I think this is since 99.4 might be a major update that is taking long to develop, and if it isnt then pls release it as early as possible." and read into that last part, please release it as early as possible (in the case of it being a minor update) and was wondering if you were desperate for a minor update patching one or two obscure bugs and adding compatibility with an OS that was depricated 12 years ago (made up examples of minor update content).

What is interesting about TPT is the devs are not paid. They have jobs and personal lives and so there is no real time when they all get together and have dedicated work time. Often, its more like, yea I have some time right now, let me work on X and such. As a consequence of this, urgent things are prioritized, after that, things they find interesting/fun to work on, and after that, boring, painful code that nobody will know they changed but makes the webpages just a little faster, or helps one specific type of device have a better interface, or whatever. So lacking any urgent bugs that need fixing, and (judging by the discord) not a lot of other things that need doing either, they are putting most of their time and effort into some major stuff.

As I am not a dev, mod, anything like that, this is sorta a generalization. As an example, TPT android users can now, or soon will be able to, open saves (and maybe stamps) from discord and the TPT app will start up and load it. This doesn't affect the PC/Console, but still represents something the staff were doing somewhat behind the scenes. 
https://discord.com/channels/311697121914912768/939734727265488896/1396262251857580032

This is a link to the video clip of that, and its in the official discord's #development channel. This channel has notifications when the status of pull requests is changed or when they are first created. (Pull requests are when users take the game's code, edit it, and then ask for it to be merged into the game). It ranges from highly technical discussions between developers and mods about backend server stuff, and sometimes its more broad discussions that help shape future development (such as discussing what a new element might do). Im sharing this as it seems you are curious about whats in the works, so I highly recommend you check it out.