Is it possible to create a 128-bit UUID from a weak entropy source?
The entropy will never go higher than the amount you put in. So if you require 128 - 4 = 124 bits and you input 32 bits, you can rest assured that the amount left is at most 32 bits. It's really that simple. And in that case, there is a high likelihood of collisions due to the birthday bound.
Now generally you will likely not find dupes during regular use, but an adversary may simply try and see if a collision can be made. In that case all bets are off, because in the end the RNG is simply not cryptographically secure.
I'd also be concerned about the entropy source. If it is 32 bits taken from the system clock then it is really not entropy at all, to name just one common option.

Try and avoid generating UID's at the client side entirely.
If this is really something you require then there is subtle crypto now for JavaScript in browsers, use that cryptographically secure random number generator instead. Beware of dragons though: &quot;There is no minimum degree of entropy mandated by the Web Cryptography specification.&quot; ... this might especially be an issue on non-standard / embedded browsers.