There are CATIA v4 to UG translators available from UGS. It will supposedly translate surfaces & solids into UG (history, parameters & associativity are NOT retained). When the translator was first released in NX 1, it was included on a limited time basis. I think you had to use File -> Open, change the file type manually by typing in *.model in the file name input field, then UG would filter for v4 files.
I personally never got anything to successfully translate, but that may have been due to version issues. The translator was version specific (meaning it would open only up to certain V4 RXX models).
There are some other options, but that is usually best answered by your UGS sales rep.
My experience with translations from catia to UG are not very positive. Oddly enough translations from CATIA to I-DEAS work very well. The next conversion from I-DEAS to UG works also well thru parasolids.
Success depends very much on the quality of the CATIA models. I suppose the catia i-deas translator works well due to ford motor company plants having both packages.
You'll have to know the specfic release of v4 in order to see if you will be able to translate directly into UG. I cannot remember it at the moment, but it is in the NX1 documentation as far as up to which v4 release it will support. What I think really doesn't matter, as your success is going to vary from model to model. I have no idea how complex your models are, or what sort of quality you need in your models. I have no clue if you're able to repair translated models once they are brought into UG. You will have to experiment & find out what works best for YOU & the parts you're creating/translating. Try removing complex or "heavy" features. Find out what kinds of settings CATIA has to make the IGES/STEP files easier for UG to read.
We routinely receive CATIA models from Honda in IGES format. Usually all the surfaces come in just fine into UG but there are also usually surfaces that need to be cleaned up a bit before sewing them into a solid. It all depends on the ORIGINAL data that resides in CATIA. If that data is sloppy with loose tolerances, then you're going to have to use loose tolerances in UG. It's not going to matter which translator you use if the models are sloppy or modeled at loose (high) tolerances (like 0.05 mm)....not even with STEP, as UG will loosen the tolerances in order to rebuild a solid coming from a STEP file.
If you're seeking DIRECT translations, but no associativity retained, then there are 3rd party softwares available for this. You MUST contact a UGS sales rep in order to find out cost, how to use, etc. I'm sure they would provide a temporary license upon request.