If you could send a picture (or video) of your current robot, that would be great, as it would help us help you. I (and others here on the forum) would be happy to give you suggestions, but what you are asking is a bit of a broad subject that there isn’t one concrete answer for.
As for your question about gears or chain on the 4-bar though, I’d suggest not using chain because gears allow for much more precision and take up less space.
While I do agree with this, I want to point out the specific wording that was used. When StuartV says “changing”, they mean that it could be making it faster, but it could also mean making it slower. To start with, I would suggest a 2:3 gear ratio (i.e. the motor spins twice for every three times the wheels spin).
One last note is that another way you can make your robot faster is to minimize friction. This means making sure the chain isn’t too tight, and making sure nothing is rubbing on anything else.
By a 2-bar, I’m going to assume you mean a single jointed arm, such as is used with the VEX IQ Clawbot. And by a 4-bar, I’m going to assume you mean something like what is used in Linq, Stretch, Flex, Rise, or Clutch.
As far as lifts in general go, I’ve found a 4-bar to be very effective for moving something up and down vertically without tilting it. Rise Above and Next Level are good years to look at for examples of 4-bars in action because nearly everyone had one those years. For more information on how 4-bars work, check out this video on them. It also provides some critical details on what to look out for when building one.
But every lift has its place, and 2-bars, or single jointed arms, have their uses. Although less common in VEX than a 4-bar, they are used to raise something vertically and rotate it in the process. In Squared Away, some cube-only robots used it to put it on the platforms while staying in sizebox. The standard Clawbot build has a good example of a single jointed arm though.
If you could give some details on what you are using it for, I could give you a more specific answer, as my answer above was very generic.