I type a letter and I get another. I tried playing with the letter wheel but it keeps doing same thing.

The print hammer is striking between the letters. Two thing
you may have a petal, letter broken off. If its the same letter every time usually thats it, need new daisy wheel.
Or Homing is off when the wheel turns hammer hits between letters. Needs repair.
Or
Daisy wheel petals are bent.

There is a round jockey wheel that turns the tape wheel, take of top cover off by finding screws, either buy a new jockey wheel, or boil the wheel in water for 10 minutes to make rubber soft.

Hammer sticking and typing incorrect letters (and or skipping letters): 1) make sure daisy wheel installed correctly. 2) hold down CODE key & Q (RESET) to reset wheel...if still not corrected 3) WD40 the hammer at the base & joints where moving parts are, keep typing until oil seeps in (this last one fixed it for me)

Typing light and inconsistant-top part of letter okay but bottom too light or not there: to left of hammer (at base) are 2 screws, I losened them and adjusted hammer to different forward and backward spots..finally found the ideal location where full letter printed and had the correct force it needed to print correctly.

Sounds like the daisy wheel is loose. Wiggle the daisy wheel side to side. there should be little movement. If it is loose, remove it and reinstall it. also, look for foreign objects down in the wheel printhead assembly.

If that doesn't fix it, remove the daisy wheel,unplug machine and let it rest for 20 minutes, or so. . reinstall the wheel and see what happens.

The SX 4000 is a "Daisy Wheel" printer. A plastic Wheel with the letters embossed on them spin via the input of the keyboard. I would suggest that your Daisy Wheel mhas slipped its index, presenting a offset letter., or the wheel is on inproper, but I do not see how it could happen, short of forcing it on. You may have to buy another Daisy Wheel, if that one is broken at the hub. See:

http://www.google.com/products/catalog?rlz=1T4SUNA_enUS250US259&q=daisy+wheels+for+brother+typewriters&um=1&ie=UTF-8&cid=5918209752341457784&ei=aqEHTeebDs6s8AaP-9nnAg&sa=X&oi=product_catalog_result&ct=result&resnum=4&ved=0CDEQ8wIwAw#

Please check the ink ribbon. There are two types of ribbons. The plastic one can be used once and if you try it the second time then there is no guarantee that all letters will appear fine and legible. (as may be happening in your case). The other is the cloth ribbon. This can be reused and it keeps fading as you keep repeating it. But in this case all letters are faded and nor selective.

You might have a "proportional space" printwheel but the pitch selector is on 10, 12, or 15 instead of PS. OR it is on PS but you have a normal print wheel. Try selecting different pitches. Another thing that causes that problem is sometimes the ribbon or correction tape gets caught in the printwheel.

there is a slide switch on the L.H. side of the key board that reads 10,12,15, PS
the switch needs to match the print wheel stile you have in your typewriter. most typ's have 10 or 12.
PS stands for proportional spacing a takes a spicule print wheel

I downloaded the manual from http://www.devicemanuals.com/downloadguide.php and then, following the instructions there, I removed then re-inserted the daisy wheel. This solved my problem. Perhaps it had somehow gotten out of adjustment. At least, it is something to try if you haven't tried it already.

Does it always type the same wrong letter ie a always prints u or w ? if so the print wheel drive gear is out of position. if not and it get random letters some thing may be interfering with the wheel rotation.