Softmods and Wii System Menu 4.3 are two separate things. You need to do a little reading, as we all do when we start off in this hobby. I great resource that has helped me is the Complete Softmod Guide, and a great utility to use after reading the Complete Softmod Guide is ModMii. If you have any questions after reading that can't be found with a search of these forums, feel free to ask away.
Welcome to the hobby, and enjoy your new red Wii
EDIT: Darn you mauifrog, beating me to the post, lol. Well there is another great guide to read as well
I would actually suggest using ModMii; a quick search for ModMii on these forums will find you the download
but it's the easiest to put together since all you do is run it, it'll ask you a couple of questions and download everything for you. All you need is the games Super Smash Bros. Brawl, Lego Indiana Jones, or YuGiOh 5D's. once you get that done your good to go
So basically all you need is 1 of 3 games and ModMii to softmod your Wii nowadays.
I noticed on his guide it says DO NOT EVER UPDATE YOUR WII.... Well this is partially true. You should never do a Official Nintendo Update! As this will most likely brick you. If you insist on having the latest System Menu @ all times and IOS you should look into Dop-Mii Wiibrew Edition. You can simply install the newest System Menu itself or any other IOS. The purpose of updating this way is that you will NEVER EVER AGAIN HAVE TO REDO YOUR SOFTMOD!
it's safe and reliable; you can choose WAD Installation or Network Installation. If you want to do WAD Installation you can get all your IOS off ModMii even your System Menu WAD file. Another benefit to this is when installing the new System Menu with this homebrew app it will alert you if you have the correct IOS required to run that System Menu and ask you if you would like to install it.
Brick Protection. People often brick upgrading System Menu's because they done have the correct IOS installed for that particular System Menu.
Network Installation is well will download all the files w/o you ever having to transfer them to your sd card and once they are installed the files are discarded.
But to answer your question more clearly you can get System Menu 4.3 firmware off ModMii. Pm me if you like to know a little more bassicly it cant get any more simpler then having one of those three games and download a pc app called ModMii
You should also install Priiloader and Bootmii
You can get Priiloader off google.code or search wiibrew.org "Priiloader" w/o quotes.
Google Bootmii to find that site