I have an electrical panel in my garage with a single row of breakers 4 single 15A breakers installed and 4 blank spaces left. (pretty certain it is a sub-panel off the house) It has a sticker that says 70A max. Am I able to install a 50A 240v breaker in the spaces left over?
There is not enough information to give a yes or no answer. The breaker that supplies this panel and the wire size would be needed to say if it would work. Can a 50 amp breaker be installed in a 70 amp panel yes. Can this be done if the total of all the breakers add up to more than the panel rating? Yes but the over current protective device (breaker) feeding the panel can not be more than 70 amps. One easy example would be for a heating and cooling sub panel lets say 60a of electric heat and a 30a AC unit total 90 amps but it is legal because only 1 load would be used at a time. So in short a 50 amp breaker can be used in a 70 amp panel but if your breaker feeding the panel or the wiring is not sufficient size it won't work.
[sub-panel] has a sticker that says 70A max. Am I able to install a 50A 240v breaker?
Or at least, you need to check several other things before installing your 50A breaker.
The sticker is probably the manufacturer's maximum rating for the sub-panel itself.
What mostly counts is the gauge of wire from the main panel to the sub-panel. This might not be able to carry the full 70A safely.
The rating of the related breaker in the main panel is also a limiting factor.
USA or other 120V split-phase country
If the current panel contains only 120V breakers, you are unlikely to be able to provide 240V service there. You might need to replace a single-pole breaker in the main panel with a double pole breaker and run new 4-conductor cabling from there to the sub-panel.
If the sub-panel already provides 240V service, the voltage isn't an issue.
UK or other 230V country
The voltage isn't an issue.