Antenna for 4G USB modem

Does anyone have any recommendations for an antenna for a 4G USB modem?

First, I'd start with some good pigtails. Any gain in the antenna will be offset by losses in the pigtails and any feed line you use. The tiniest of cable typically used has significantly more loss than 1.32 cable. I buy mine through a major US-based electronics component supplier and have little confidence in the eBay units (especially those using 1.13 or unspecified cable). My last order was with Mouser, this for a 100 mm, SMA(F) pigtail (check if your antennas are "normal" or "reverse" polarity (RP-SMA), as well as male or female).

If you can skip feed lines completely, that would be strongly suggested. Unless you're willing to spend US$50 or more for a couple meters per feed line, you'll likely lose more in the feed line than you'll gain from the antenna itself, especially once you get up to 2 GHz and above.

Similarly, I'd skip anything with an attached feed line. (An exception here might be a 100-150 mm integrated feed line for a "patch" style antenna intended for direction connection to the LTE module)

I run a pair of "700-2600Mhz 5dbi 4G LTE Rubber Duck Antenna with SMA male straight" antennas purchased on eBay from an apparently reputable RF supplier. They're comparatively big, about 300 mm long. No matter how aesthetic having them stick straight up might be, having them at right-angles to each other will likely improve your receive diversity. Left/right tilted 45° each, one upright the other horizontal, whatever...

I've also used a rectangular patch-style antenna, about 20 x 110 mm that was very functional and might be "just fine" if you're in a location where you get good coverage on your cell phone. However, as we're in a low-signal area, the gain and diversity of the two "crossed" antennas is noticeably better.

All the gain and VSWR specs all seemed to be about the same across antennas I looked at. I wouldn't trust them anyways. Assuming similar technologies for antennas and their construction, generally "bigger is better" with 2x the length (or comparable width for some of the patch-style antennas with right-angle feed) probably being noticeable with these kinds of antennas.

Check the bands used by your carrier, as well as those available on your modem. Then make sure that the antenna has frequency coverage over those bands. For example, T-Mobile US is heavily deployed here on Band 12 at 699-746 MHz. Many antennas don't cover the low bands.