# USM lock C vs Zeliox ZAI4

The clearest difference is update cadence: USM lock C at every 3 months against Zeliox ZAI4's every 7 months. Zeliox ZAI4's advantage is iOS requirement (12.0 vs 15.6). On price, ads, in-app purchases and device support there is nothing between them.

Both are free to download. Neither carries in-app purchases, so what you see is what you pay.

USM lock C ships an update every 3 months, Zeliox ZAI4 every 7 months. The most recent releases landed on September 19, 2026 and September 23, 2026 respectively.

## The numbers

| Metric | USM lock C | Zeliox ZAI4 |
| --- | --- | --- |
| Price | Free | Free |
| In-app purchases | No | No |
| Ads | No | No |
| Update frequency | **Every 3 months** | Every 7 months |
| Size | 47 MB | 34 MB |
| Requires iOS | 15.6 | **12.0** |
| Devices | iPhone, iPod | iPhone, iPod |
| Age rating | 4+ | 17+ |
| Developer | [USM U. Schaerer Soehne AG](https://appshunter.io/developer/395765701) | [Zeliox](https://appshunter.io/developer/1478320738) |

## What reviewers say

### USM lock C

[App page](https://appshunter.io/ios/app/usm-lock-c/id6467190717)

**Features:** macOS Compatibility

**In-app purchases:** none


### Zeliox ZAI4

[App page](https://appshunter.io/ios/app/zeliox-zai4/id6737747373)

**Features:** macOS Compatibility

**In-app purchases:** none


## Frequently asked questions

### Is USM lock C free?

USM lock C is free to download, with no in-app purchases.

### Is Zeliox ZAI4 free?

Zeliox ZAI4 is free to download, with no in-app purchases.

### Do USM lock C or Zeliox ZAI4 have ads?

Neither USM lock C nor Zeliox ZAI4 shows ads.

### Which is updated more often, USM lock C or Zeliox ZAI4?

USM lock C ships an update every 3 months, and Zeliox ZAI4 every 7 months. Most recently, USM lock C was updated on September 19, 2026 and Zeliox ZAI4 on September 23, 2026.

---

*Compared by [AppsHunter](https://appshunter.io/) from daily US App Store data. Review summaries are generated from verbatim App Store reviews.*