We are building a portable 48 volt system (6000 watt output).
I'm guessing OP is referring to 6kW of inverter output, not PV input, otherwise I really would like to know about their 6kW "small solar panel setup that travels well".
In my search for a voltage-boosting MPPT charge controller, I ran across
this BougeRV model, which is actually a buck-boost controller with the widest "low-voltage" input range I've yet seen on an MPPT, with 25 Amp PV input and 20 Amp charging output. Unfortunately I can't yet find any reviews or even mentions of it anywhere, really. The manual also states that there is a 40 Amp version capable of handling 50 Amps of PV input, but Google has never heard of the 40 Amp version. The 20 Amp version is also
sold on Amazon, but it's a multi-product listing and
zero of the posted reviews apply to this buck-boost controller.
For OP or anyone wanting the ability to use from one to just a few panels to charge, this buck-boost MPPT would seem to be just the ticket, but that precludes the simplicity of routing everything into an all-in-one inverter like the EG4 6000XP that was mentioned, and would mean piecing together a custom setup with more components.
The manual says the input of an EG4 6000XP needs to see a minimum of 120 volts PV to wake up the charger when charging a 48 volt battery. That's around 1200 watts.
I'm not seeing where you got the 1200 Watts number, but as regards specifically the EG4 6000XP (to my understanding as a noob--someone please correct me if I'm wrong), at the minimum of 120 volts PV input, along with anything between greater-than-zero current all the way to the max current rating (17 Amps), the unit
will charge the battery, albeit very slowly for low PV input current. The 6000XP would charge the battery at a maximum of 2040 Watts if you were somehow able to feed it the max 17 Amps of PV at the minimum 120V PV, but by my calculations that would look something like having between 3 and 4 strings of serial-connected typical (~24V Voc / ~5 Amp) 100 Watt panels with 5 panels per string, or in other words 15 to 20 panels @ 100W each.
The only thing I know of that might work to stick with something like the EG4 6000XP or similar and still charge with just a few ~100W typical panels would be to put a DC-DC boost converter on the PV line between the panels and the MPPT input of the all-in-one inverter, but for one thing I can't find any with the proper input and output ranges that look like they're not total junk, and for another thing I'm guessing other forum members could give me a thousand reasons why that's a bad idea.