Theres broadly 3 categories of signal level when recording audio;
Instrument / mic - Lower level, more complex signals. They are actually very different from each other but both are signals that require a specific type of input to record properly. Mic signals need a LOT of preampfification, instrument signals need some pre-amplification (though different instruments have different needs, instrument REALLY just means guitars as most electric keyboards output a simple enough signal to be considered more or less line level). Instrument signals typically need a higher impedance input to match or ideally be way more than a guitars output impedance. This is why you often use a DI, a passive DI doesn't amplify a signal but prepares it to be suitable to go into a mic preamp. A guitar is a bit too high impedance and a bit too low level to go directly into a line level input, but a DI deals with the impedance and sets up the signal to be good to go into a mic pre, which will provide the needed gain.
Line - Line level is the basic working standard for passing audio between bits of audio gear. It is utilitised such that a line output will be low enough impedance to properly drive any line input, and the signal will be of an appropriate strength (voltage in this case). The confusing point is that there are two standards for line level, -10dBV and +4dBu, generally termed consumer and pro level. This is not really used TOO much these days, it's a bit of a throwback to earlier gear.
Speaker - Speaker level is a whole different thing, it's electrical rather than electronic, that is, it's the converted signal in a format that can do the work needed to correctly drive the speakers. It's usually higher voltage than line and can deliver a LOT more current.
So inputs 3 and 4 on your interface are expecting to receive a line level signal, preamplified. An electronic keyboard would be fine going into them, as would any output from a guitar preamp or similar. You may get ok results just plugging a guitar straight in, but it would probably need a fair bit of gain. A buffered pedal with gain on it would probably do the job ok too.
Be very careful of plugging a speaker output into any small signal device such as a sound card, the speaker outs would easily have the power available to completely overload the input and maybe even burn something out, it's just a general no no to plug a speaker out into anything but a passive speaker. This is electricity as a driving 'energy' at this stage, not just a signal, though if you know what you are doing you can attenuate and capture a speaker out signal. I would generally just say DON'T do it though.
The Morley box you link is interesting, I would be curious to see its internals. It is just converting between the 2 different line level standards, probably using transformers to step up / down the voltage. At line level (either 'consumer' or 'pro') the signal is close enough that you could move between the two ok with a device such as the Morely. However I don't think it would give you what you need to record a guitar, certainly not a microphone. It is designed to convert between two differing standards of line level, not to go from instrument / mic level to line level. Although it may actually work OK for a guitar, as a line input on max gain will usually at least capture a guitar signal ok. With the extra boost from the Morley you may get something workable, but it's far from 'correct'.
Best bet to use inputs 3 & 4 would be to get SOMETHING that boosts to line level. A cheap mic pre for a mic and anything powered, with a gain control, for a guitar, ideally something that has a line level out. The line inputs on a device are designed to allow this flexibility. It's mic / instrument pre's that cost money, the processor in the focusrite likely has the spare inputs to process 2 more line audio streams and they have made this (designed this) to be available on the back panel, but you need a line level signal, from a mic pre, guitar pre, mixer aux or direct output, electronic keyboard etc. to PROPERLY use them.
