No modifications are possible with the 4620L or any other MiFi model. You get what you get when it comes to antenna ports and it would not be wise to modify them otherwise. Chances are that your antenna does not support 4G LTE anyways unless you specifically purchased it to do so. Most antennas and ports are only compatible with 3G technology.
Instead of a wired antenna I would suggest using a wireless cradle for your MiFi. Products do exist that will act like a wireless booster in your car, home or business. The only down side is that these boosters are powered so you need to plug them into something to use them.
I bought both a RF booster that is specifically designed for Verizon 4G LTE (AT&T's 4G is different by intent) and a new antenna for the 4G spectrum used by Verizon. A wireless cradle will boost the signal but not provide the gain in signal of an external antenna. With any radio transceiver the antenna and its height above the horizon has a far greater impact on reception than the amplifier alone can provide. Cradles are fine for a quick and dirty partial solution in a vehicle but it is worthless in a building and hardly optimum for use on the road.
Any device can be modified and there are many thousands of people that have hacked the iPhone to fix its defects in this same regard. There are cell phone replacement antennas sold that take the place of the small ones in certain models and provide a 8" collapsible one in its place.
Wireless boosters have repearter functions and per the FCC these are going to be illegal to use starting in 2014 unless you receive written permission from the ISP - Verizonwireless in this case. Based on the past performance of the telephone companies this is going to shut things down for a year and then the booster they approve are going to be twice as expensive. That is why they all are using different flavors of 4G so you are locked into their phones and hotspots and their signal boosters as well as their 2-year contracts. Small wonder the USA is ranked number 19 in internet access and performance and dropping.
Sounds like the FCC has pretty loose rules on consumer grade wireless boosters. All you have to do is register them with the ISP and you will be fine. If the ISP or FCC contacts you about a problem you can work it out and correct it with them at that time.
Where the FCC is cracking down is the industrial use cell phone boosters. Those will require special permissions to purchase and install. Fees for violations are posted and pretty hefty too. The way I read it the FCC does not say that they will be illegal, you just have to clear it with them or the ISP first to make sure there is no interferance.
FCC Booster Rules adopted on Feb 20, 2013:
You can use a Cellphone-Mate SureCall 70db Wireless Amplifier for Verizons 700 MHz 4G LTE
Read some of the reviews about this product I think it can help with your Verizon MIFI in a building type situation to get you a strong signal
According to the FCC ruling the ISP has to approve the device you use and do so in writing. Anyone who thinks that this is going to just happen and the phone companies are going to do what is best for the customer has not been around for the past 40 years. When AT&T controlled what could be attached to their network the best one could do was a 300 baud modem from AT&T that clamped to the phoneset. Anything else was forbidden.
Now we have 4G LTE from Verizon and AT&T that are incompatible with each other and that tells me the direction we are headed in the USA. The FCC has never acted in the public's interest since its inception. The AT&T breakup and similar loosening up of the public's airwaves (that the telephone companies are using) were done through the courts with judges leading the way. Absolutely no reason to naively assume it is going to change.