If I want to dual boot with XFCE and KDE using BTRFS do I need to install using EFI?
No, works well with either UEFI or the older legacy method.
But in Legacy mode it will use more than 4 partitions, I thought that was the max?
They are not partitions, they are sub volumes.
Please note that the “regular” os-prober will not detect other systems installed with btrfs on your disk(s). That means you would not get a boot entry for the other system on the Grub boot menu.
If your system supports UEFI, then why go for mbr/legacy install?
Legacy mode is easier to setup. But It troubles me If I can’t select KDE or XFCE in the grub menu
I think the opposite 
You need the os-prober patched by Manjaro. That is what I use in my multi-boot system where all the systems are installed with btrfs. But mine are all in UEFI as well. Please see:
If you have one of the four primary partitions set up as an extended partition you can have many logical partitions inside of the extended partition which all could carry a bootable Linux system.
@Alexander
Why do you want to use legacy Bios over UEFI?
Ease of setup
If you use the wiki that @2000 posted it is very easy. You can do encrypted or unencrypted installation.
Do you have a link?
It’s here on the wiki; This is for Encrypted installation of BTRFSonLUKS with timeshift and snapshots. There are two versions. One is a quick copy & paste and the other is the verbose mode. (Manual Install)
You can also install it without encryption.
Here are the changes if you want to do unencrypted method.
It’s different nowadays with modern computers and better implementations, but UEFI was a major PITA in the past. I used legacy mode exclusively until 2019.
Still not a fan of UEFI, as it doesn’t give me any real advantage on my setups.
When in the past are we talking? 2005-2007
Edit: I like UEFI better now.
I can’t tell you an exact year, but my misadventures were around 2012 or so.
But I don’t have any trouble with it anymore on recent hardware, and yes, on new-ish hardware EFI is usually a better choice.
Still, I think Legacy is still a valid option, it should IMO not be disregarded.
I think Bios will be around for some time but it does have some limitations hence why UEFI was implemented. I still think some manufacturers do not do a good job implementing UEFI and standards are not always adhered to as with anything. A lot of these companies want to implement proprietary versions so others can’t use it and it’s all about $. I like the fact that there are some such as system76 who are working on open source hardware and core boot.
That was actually the reason why I used Legacy. The implementation was simply bad (problems with sound, longer boot time).
Anyways, OP’s question has already been answered, sorry for semi-offtopic 
Are the advantages of using UEFI big enough for me to reinstall?
I would go UEFI if you have it as opposed to Bios.